Glossary of Terms

Glossary of Terms

When reading about eye health and care, there are quite a few scientific words and phrases you may be encountering for the first time.  For your use as a quick and easy reference, we've compiled an alphabetical glossary for you that offers definitions and explanations.


Aberration (ab-ur-AY-shun)
Blurred or distorted image quality that results from inherent physical properties (shape, curvature, density) of an optical device (lens or prism).


Scraped area of corneal surface, accompanied by loss of superficial tissue (epithelium).


Accommodation (uh-kah-muh-DAY-shun)
Increase in optical power by the eye in order to maintain a clear image (focus) as objects are moved closer.
Occurs through a process of ciliary muscle contraction and zonular relaxation that causes the elastic-like lens to round up and increase its optical power.


Measure of an eye's ability to distinguish object details and shape. Assessed by the smallest identifiable object that can be seen at a specified distance usually 20 ft or 16 inches.


AK (astigmatic keratotomy)
Method of reshaping the cornea to correct astigmatism.
Small incisions are placed in the corneal periphery parallel to the limbus and perpendicular to the steepest meridian, to flatten that meridian.


Medication that removes all sensation, including pain.


Class of medications (many derived from bacteria or molds) that controls or eradicates bacteria that invade and grow in the body, causing infections.


Amblyopia (am-blee-OH-pee-uh)
Decreased vision in one or both eyes without detectable anatomic damage to the retina or visual pathways. Usually uncorrectable by optical means (e.g. eyeglasses).


Method of flattening the cornea. Used for measuring intraocular pressure.
See also tonometer.


Artificial tears
Eyedrops that approximate the consistency of normal tears. Alleviates dry eye symptoms; some are used for treating recurrent corneal erosion.


Type of ultrasound; very high frequency sound waves that are reflected by the ocular structures and converted into electrical impulses. Used for measuring length of eyeball (axial length) prior to cataract surgery, to help determine power of IOL to be implanted; also to help differentiate normal and abnormal eye tissue.


Astigmatism (uh-STIG-muh-tiz-um)
Inability of an eye to focus sharply (at any distance), usually resulting from a spoon-like (toric) shape of the normally spherical corneal surface. Instead of being uniformly refracted by all corneal meridians, light rays entering the eye are bent unequally, which prevents formation of a sharp focus on the retina. Slight uncorrected astigmatism may not cause symptoms, but a large amount may result in significant blurring. Corrected by a cylindrical (toric) eyeglass or contact lens, or refractive surgery.


Bandage lens
Soft contact lens with no refractive power, used for protecting damaged or irregular corneal surfaces.


BAT (brightness acuity test) Glare test
Determines the extent to which bright light shining in the eye changes a patients visual acuity. Useful for evaluating degree of visual impairment caused by a cataract.


Best-corrected visual acuity (BCVA, BVA)
Vision obtained with best possible lens correction.


Eyeglasses that incorporate two different powers in each lens, usually for near and distance corrections.


Blepharitis (blef-uh-RI-tus)
Inflammation of the eyelids, usually with redness, swelling and itching. Many causes, e.g. infection, allergy.


Blepharoplasty (BLEF-uh-roh-plas-tee)
Any plastic surgery of the eyelids; usually cosmetic.


Blue blockers
Refers to a type of lens coating or incorporated chemical that selectively absorbs short wavelength (blue) light rays.


Trade name of botulinum toxin-A. Injected into muscles for temporary paralysis; alternative or addition to surgery to correct eye misalignments or to paralyze a facial nerve causing uncontrollable lid spasms.


Cataract (KAT-uh-rakt)
Opacity or cloudiness of the crystalline lens, which may prevent a clear image from forming on the retina. Surgical removal of the lens may be necessary if visual loss becomes significant, with lost optical power replaced by an intraocular lens, contact lens, or aphakic spectacles. May be congenital or caused by trauma, disease, or age.



Clear lens extraction (CLE)



Conjunctivitis (kun-junk-ith-VI-tis)
Inflammation of the conjunctiva (mucous membrane that covers white of eye and inner eyelids.) Characterized by discharge, grittiness, redness and swelling. Usually viral in origin; may be contagious.


Contact lens
Small plastic disc containing optical correction, worn on the cornea or sclera to correct refractive error or to protect the cornea.


Refers to a condition that suggests that a given treatment or procedure could be harmful and should not be done.


Contrast sensitivity
Ability to detect detail having subtle gradations in grayness between test target and background. Tested with specially designed targets or cards.


Cornea (KOR-nee-uh)
Transparent front part of the eye that covers the iris, pupil and anterior chamber and provides most of an eyes optical power.


Corneal Refractive Therapy (CRT) Paragon CRT
Use of rigid gas-permeable contact lenses during sleep to flatten the cornea; permits a modest temporary correction of myopia during the day without glasses or contacts.


Corneal transplant


Corneal topography
Map of the variations in front surface curvature of the cornea, much like making a contour map of land.


Corneal ulcer
Area of epithelial tissue loss from the corneal surface. Associated with inflammatory cells in the cornea and anterior chamber. May be caused by bacterial, fungal, or viral infection.


Counts fingers (CF)
Patient's ability to count number of fingers presented, usually at a distance of 1 or 2 feet. Administered when vision loss is profound acuity less than 20/400.


Cross-eyes esotropia (ee-soh-TROH-pee-uh)
Eye misalignment in which one eye deviates inward while the other fixates normally.


Cycloplegic refraction (CR)
Assessment of an eyes refractive error after lens accommodation has been paralyzed with cycloplegic eyedrops (to eliminate variability in optical power caused by a contacting lens).


Debridement (deh-BREED-ment)
Removal of dead or infected tissue and foreign material, to aid in healing.


Depth perception
Awareness of the relative spatial location of objects, some being closer to the observer than others.


Diabetic retinopathy (ret-in-AHP-uh-thee)
Series of progressive retinal changes accompanying long-standing diabetes mellitus. Early stage is background retinopathy (non-proliferative). May advance to proliferative retinopathy, which includes the growth of abnormal new blood vessels (neovascularization) and accompanying fibrous tissue.


Diffuse lamellar keratitis (DLK) Sands of Sahara syndrome
Superficial clouding of the cornea under a corneal flap, with a granular-looking interface (between the surfaces), due to inflammation. Complication of LASIK that is usually reversible.


Dilate (DI-layt)
To widen an opening, such as the pupil or lacrimal punctum.


Diopter (D)
Basic unit of lens refractive power; equal to the focal length (in meters) of a lens, e.g. a 2-diopter lens brings parallel rays of light to a focus at ½ meter. Also a measure of the degree that light converges or diverges, i.e. the reciprocal of the distance between an object or image and any reference plane.


Disc optic disc, optic nerve head
Ocular end of the optic nerve. Denotes the exit of retinal nerve fibers from the eye and entrance of blood vessels to the eye.


Distance vision
Visual acuity measured with the target of 20 fit. (6 m), the optical equivalent of €œinfinite distance.


1.  Imperfectly shaped image created by an optical system of lenses or mirrors.

2.  Perception created by a wrinkled or irregular retina.


DLEK (deep lamellar endothelial Keratoplasty)
Type of corneal transplant in which only a segment of the deepest layer of donor tissue is used to replace a patients damaged endothelium, without sutures.


Dominant eye
1.  Preferred eye for various visual tasks, e.g. sighting.

2.  The eye that leads and controls the other during binocular eye movements. Usually on same side of body as dominant hand.


Dry eye syndrome
Corneal and conjunctival dryness due to deficient tear production, predominantly in menopausal and post-menopausal women. Can cause foreign body sensation, burning eyes, filamentary keratitis, and erosion of conjunctival and corneal epithelium.


Ectasia (ek-TAy-shuh)
In the eye, refers to a thin, stretched sclera or cornea, as can occur after congenital glaucoma or inflammation of the outer wall of an eye.


Edema (eh-DEE-muh), oedema
Swelling of tissues from excess fluid accumulation.


Enhancement, retreatment Surgical fine-tuning of an earlier procedure some time later, usually after some loss of refractive correction.


Enucleation (ee-nu-klee-AY-shun)
Removal of the eyeball, leaving eye muscles and remaining orbital contents intact.


Enzymatic cleaner (en-zih-MAT-ik)
Tablets (dissolved in water) that remove protein and deposits from the surface of soft contact lenses. Used in addition to cleaning and sterilization.


Similar to LASEK, except that a microkeratome (instead of alcohol) is used to lift the epithelium, which is folded back. An excimer laser reshapes the corneal surface, then epithelium is returned to its normal position. Corrects myopia, hyperopia and astigmatism, especially with thin corneas.


Epithelial ingrowth, epithelial downgrowth
Abnormal growth of surface epithelium into the eye through a penetrating wound or eye surgery (e.g. cataract). Rare, serious complication; epithelium covers internal structures. Also a complication of certain refractive surgery procedures (e.g. LASIK), in which epithelium grows underneath the corneal flap.


Epithelium (ep-ih-THEE-lee-um)
Membranous multicellular layer covering the internal and external surfaces of the body and its organs. In the eye, covers the cornea, conjunctiva and eyelid. Corneal epithelium: outermost layer of the cornea, between Bowmans membrane and the tear film.


Erosion (recurrent corneal)
Episodic, periodic loss of the outer layer of cornea (epithelium) due to its failure to adhere properly to Bowmans membrane. May follow minor scratch-type injury.


Excimer laser (EKS-ih-mur)
A cold laser (filled with argon-fluoride gas) used in refractive surgery procedures to reshape the cornea. Controlled by computer, the laser emits a pulsating ultraviolet light that can ablate (vaporize) corneal tissue to a precise depth and area to produce a given optical correction, without heating it or surrounding tissues. Results in a smooth surface that heals rapidly with minimal scarring.


External disease
Refers to diseases that affect the cornea, sclera, conjunctiva or eyelids.


Eye, eyeball, globe, bulb
Sense organ for sight. Receives light imagery and transmits the visual information to the brain. Composed of three major structural layers (corneo-sclera, uvea and retina) and includes the lens, aqueous and vitreous.


Eye examination
Evaluation of any or all of the following: visual acuity for distance and near (with and without correction), intraocular pressure, pupil function, checks for external and internal infection, disease or defects, extraocular muscle function, inspection of lens and retina through a dilated pupil.


Eyelids lids
Structures covering the front of the eye, which protect, it, limit the amount of light entering the pupil, and distribute tear film over the exposed corneal surface.


Discomfort while using the eyes for visual tasks.




Femtosecond laser
Clinical laser that produces an extremely short pulse of photons (light beams). Can be used as a microkeratome to make corneal flaps in refractive surgery, or in the laboratory to probe chemical reactions, such as the breaking of bonds between atoms.


Tongue- or disc-shaped section of tissue that is dissected on 2 or 3 sides and reattached.


Flashes flashers, lightning streaks
Sensation of light often due to mechanical stimulation of the retina from a newly formed retinal tear, the tugging of a vitreous strand, or bumping of a detached vitreous body. Also a symptom of ophthalmic migraine.


Floaters vitreous floaters
Particles that float in the vitreous and cast shadows on the retina; seen as spots, cobwebs, spiders, etc. Normal; also occurs with aging or with vitreous detachment, retinal tears, or inflammation.


Fluorescein angiography (FLOR-uh-seen an-jee-AHG-ruh-fee)
Used for evaluating retinal, choroidal and iris blood vessels, as well as any eye problems affecting them. Fluorescein dye is injected into an arm vein, then rapid, sequential photographs are taken of the eye as the dye circulates.


Gas permeable lens rigid gas permeable (RGP) lens
Rigid plastic contact lens that allows oxygen and carbon dioxide penetration.


Undesirable sensation produced by brightness that is much greater than that to which the eyes are adapted. Causes annoyance, discomfort, or loss in visual performance. May be a complication of refractive surgery.


Glaucoma (glah-KOH-muh)
Group of diseases characterized by increased intraocular pressure resulting in damage to the optic nerve and retinal nerve fibers, with typical visual field defects and increased size of optic cup. A common cause of preventable vision loss.


Glaucoma suspect ocular hypertension
Patient who has elevated intraocular pressure and/or family history of glaucoma, but no optic disc changes or visual field loss. May or may not develop glaucoma with time.


Hazy ring seen around lights; can occur with a refractive error or optical defect (e.g. cataract, corneal swelling), or as a complication of refractive surgery.


Hard contact lens
Rigid plastic lens that floats on the corneal tear film. Usually made of polymethyl methacrylate (PMMA).


Diffusely scattered light reducing clear visualization.


HIPAA (Health Insurance Portability and Accountability Act)
1996 act sets rules for patient privacy, security of health information, health insurance shifts after leaving an employer. Severe penalties for non-compliance.


Hyperopia (hi-pur-OH-pee-uh), farsightedness, hypermetropia
Focusing defect created by an underpowered eye, one that is too short for its optical power. Light rays from a distant object enter the eye and strike the retina before they are fully focused (true focus would be behind the retina). Farsighted people can often see clearly in the distance but only if they use more focusing effort (accommodation) than those who have normally powered eyes; close-up vision may be blurred because it requires even more focusing effort. Corrected with additional optical power, supplied by a plus lens (spectacle or contact) or refractive surgery.


Hyperopic (hi-pur-OH-pik, hi-pur-AH-pik)


Implantable contact lens (ICL), implantable collamer lens, phakic IOL
A lens surgically placed inside the eye in front of the natural lens, to correct refractive error, (particularly high myopia nearsightedness). Allows younger patients to retain accommodative ability of their natural lens.


Body's localized protective response to injury, infection or irritation by enclosing the involved area. Characterized by pain, heat, redness and swelling.


Intraocular lens (IOL), implant, pseudophakos
Plastic lens that may be surgically implanted to replace the eyes natural lens.


Intraocular pressure (IOP)
1. Fluid pressure inside the eye. 2. Assessment of pressure inside the eye with a tonometer. Also called tension.


Intacs intracorneal ring segments, intrastromal corneal ring segments
Plastic ring segments implanted into a tunnel cut into the peripheral corneal stroma. Allows the central cornea to flatten, reducing myopia (nearsightedness) or the bulging affects of keratoconus. Reversible; rings can be replaced or removed.


Pigmented tissue lying behind the cornea that gives color to the eye and controls amount of light entering the eye by varying the size of the papillary opening. Most forward extension of the middle layer of the eye; separates the anterior chamber from the posterior chamber.


Jaeger test (YAY-gur or JAY-gur)
Assessment of near visual acuity using numbers and symbols in a graded series of type sizes.


Keratoconus (kehr-uh-toh-KOH-nus)
Degenerative corneal disease affecting vision. Characterized by generalized thinning and cone-shaped protrusion of the central cornea, usually in both eyes. Becomes apparent during 2nd decade of life. Hereditary.


Keratome (KEHR-uh-tohm)
Any instrument used for cutting the cornea. The simplest type of a flat triangular blade.


Keratometer (kehr-uh-TAH-mih-tur), ophthalmometer
Used for measuring corneal curvature (K-readings), for fitting contact lenses, and for detecting and measuring corneal astigmatism.


Corneal curvature measurements obtained with a keratometer. Unequal measurements indicate astigmatism.


Lacrimal apparatus


Lamellar Keratoplasty (LKP) (KEHR-uh-toh-plas-tee)
Partial-thickness corneal transplant where only the outer cornea layers are excised and replaced with donor corneal tissue. (Automated (ALK). Method of reshaping the cornea to change its optical power. An automated microkeratome is used to remove a cap of corneal tissue, then a thin layer is shaved from the exposed surface, followed by replacement of the corneal cap. Corrects myopia.)


LASEK (LASer Epithelial Keratomileusis)
Method of reshaping the cornea to change its optical power. Merges features of LASIK and PRK. Instead of a microkeratome, dilute alcohol is used to loosen epithelium, which is folded back. A computer-controlled excimer laser reshapes the exposed corneal surface, then the epithelium is returned to normal position. Corrects myopia, hyperopia and astigmatism, especially in thin corneas.


Acronym: Light Amplification by Stimulated Emission of Radiation. High energy light source that uses light emitted by the natural vibrations of atoms (of a gas or solid material) to cut, burn or dissolve tissues for various clinical purposes: in the retina, to treat diabetic retinopathy and macular degeneration, to destroy leaking and new blood vessels (neovascvularization); on the iris or trabecular meshwork, to decrease pressure in glaucoma; after extracapsular cataract extraction, to open the posterior lens capsule; on the cornea, for refractive surgery, to change its optical power.


Laser refractive corneal surgery
Any laser surgery on the cornea to change its optical power, e.g. LASIK, PRK.


Laser thermal Keratoplasty (LTK), holmium laser thermokeratoplasty
Method of reshaping the cornea to change its optical power. Heat is applied to the cornea with a laser to shrink the stroma, steepening the corneal curvature. Corrects hyperopia.


(Laser Assisted in-Situ Keratomileusis) Method of reshaping the cornea to change its optical power. A flap of cornea is cut with an automated microkeratome or femtosecond laser and folded back. Then a computer-programmed excimer laser reshapes (sculpts) the exposed surface of corneal tissue; the flap is replaced without suturing. Corrects myopia, hyperopia, and astigmatism.


Lazy eye


Any piece of glass or other transparent material that can bend light rays predictably.


Lens (crystalline)
Natural lens of the eye. Transparent, biconvex intraocular tissue that helps bring rays of light to focus on the retina. Suspended by fine ligaments attached between ciliary processes.


Lensometer (lenz-AH-mih-tur)
Instrument for determining the refractive power of an eyeglass or contact lens.


Limbal relaxing incisions (LRI), corneal relaxing incisions
Transverse cuts in the peripheral cornea across the steepest meridian, to decrease corneal astigmatism. See also ASTIGMATIC KERATOTOMY.


Low-vision aids (LVA)
High-powered plus lenses and telescopes with high magnification, to help patients who have poor vision.


Macula (MAK-yu-luh)
Literally, yellow spot. Small central area of the retina surrounding the fovea; area of acute central vision (used for reading and discriminating fine detail and color.)


Macular degeneration (age related) (AMD)
Group of conditions that include deterioration of the macula, resulting in a loss of sharp central vision. Two general types: dry, which is usually evident as a disturbance of macular pigmentation and deposits of yellowish material under the pigment epithelial layer in the central retinal zone; and wet, in which abnormal new blood vessels grow under the retina and leak fluid and blood (neovascularization), further disturbing macular function. Most common cause of decreased vision after age 50.


Manifest refraction (MR)
1. Presentation of a series of test lenses in graded powers to determine (without dilating eye drops) which corrective lenses provide the sharpest, clearest vision. 2. Prescription for eyeglasses or contact lenses resulting from this test.


Microkeratome (mi-kroh-KEHR-uh-tome)
Cutting device designed somewhat like a carpenters plane, used for dissecting (shaving) a precise thickness of tissue from the corneal surface. Term is used interchangeably with automated microkeratome (most models are automated). Automated: has a power-drive oscillating blade that creates a smooth corneal bed.


Micron (µ), micrometer (MI-kro-mee-tur)
One-millionth of a meter; 1/1000 of a millimeter. . (e.g. Corneal thickness is measured in microns.)


Alternate method of correcting presbyopia. One eye is corrected for distance vision and the other for near with contact lenses, glasses, or refractive surgery. (The brain ignores the fuzzy image from one eye and sees the better image from the other eye.) Binocular depth perception (stereopsis) is decreased but not eliminated.


Multifocal lens
Eyeglass lens or IOL that incorporates more than one optical power, to permit focusing at different distances.


Myope (MI-ope)
Someone who is nearsighted.


Myopia (mi-OH-pee-uh), nearsightedness
Focusing defect created by an overpowered eye, one that has too much optical power for its length. Light rays coming from a distant object are brought to a focus before reaching the retina. Myopic people see close-up objects clearly but distance vision is blurry. Corrected with a minus lens (spectacle or contact) or refractive surgery to weaken the eye optically and permit clear distance vision.


Myopic (mi-OH-pik)




Near vision, near acuity
Visual acuity measured with the target at 16 in. (approximately 40 cm), corresponding to normal reading distance.


Nomogram nomograph
Chart that displays three variables so that connecting the known values of two variables with a straight line will identify the value of the third.


No-stitch surgery
Cataract extraction technique using minimal-size incision into the eye, with lens fragmentation and removal by phacoemulsification.


OCT (Optical Coherence Tomography)
Uses a computerized instrument that creates a magnified cross-section view of transparent parts of the eye. Particularly useful for evaluating the retina: thickness, regularity, macular holes, and vitreous strand adherence.


Pertaining to the eye.


Person who makes and fits ocular prostheses (cosmetic false eyes).


Oculoplastic surgery ophthalmic plastic and reconstructive surgery
Subspecialty of ophthalmology that deals with diseases and reconstruction of the eyelids, canthi, lacrimal system, orbit and surrounding tissues.


Off-label (use)
Using a drug or equipment to treat a condition other than those that are FDA-approved or commonly recommended.


Something that is opaque. Refers to anything that blocks normal transmission of light through a transparent medium.


Ophthalmic medical assistant
Certified allied health person in ophthalmology trained to perform preliminary examinations and specialized tests. Three levels: ophthalmic assistant (COA), lowest; ophthalmic technician (COT), ophthalmic technologist (COMT), highest.


Ophthalmic migraine (MI-grayn)
Vascular (blood vessel) disorder affecting blood vessels that provide circulation to one eye. Includes temporary loss of vision in that eye. Not followed by headache.


Ophthalmologist (ahf-thal-MAH-loh-jist)
Physician (MD) specializing in diagnosis and treatment of refractive, medical and surgical problems related to eye diseases and disorders.


Ophthalmology (ahf-thal-MAH-loh-gee)
Deals with the eye, its function and diseases. Includes diagnosis and medical/surgical management.


Ophthalmoscope (ahf-THAL-muh-skohp)
Illuminated instrument for visualizing the interior of the eye.


Optician (ahp-TISH-un), dispensing optician
Professional who makes and adjust optical aids, e.g. eyeglass lenses, from refraction prescriptions supplied by an ophthalmologist or optometrist.


Optic nerve
Second cranial nerve. Largest sensory nerve of the eye; carries impulses for sight from the retina to the brain. Composed of retinal nerve fibers that exit the eyeball through the optic disc, traverse the orbit, pass through the optic foramen into the cranial cavity, where they meet fibers from the other optic nerve at the optic chiasm.


Optometrist (ahp-TAHM-uh-trist)
Doctor of optometry (OD) specializing in vision problems, treating vision conditions with spectacles, contact lenses, low vision aids and vision therapy, and prescribing medications for certain eye diseases.


Optometry (ahp-TAHM-uh-tree)
Deals with function and disorders of the eye. Includes detection of disease and some types of management.


Orbit socket
Pyramid-shaped cavity in the skull (with the apex toward the back of the head), lined by the seven orbital bones and containing the eyeball, its muscles, blood supply, nerve supply, and fat. About 2 inches deep.


After refractive surgery, a change in refractive error that exceeds the attempted correction.


Pachymeter pachometer (pak-IM-ih-tur, pak-AHM-ih-tur)
Device for measuring corneal thickness or anterior chamber depth, using the optical principle of split images.


Pannus (PAN-us)
Infiltration of the cornea (just under the surface) by abnormal blood vessels and fibrous tissue.


Penetrating keratectomy (PK) (kehr-uh-TEK-toh-mee)
Removal of a full-thickness section of cornea; part of a penetrating keratoplasty procedure (corneal transplant).


Peripheral vision
Side vision; vision elicited by stimuli falling on retinal areas distant from the macula.


Phaco- phako- (FAY-koh)
Prefix: refers to the eyes natural crystalline lens.


Phacoemulsification (fay-koh-ee-mul-sih-fih-KAY-shun)
Use of ultrasonic vibration (from irrigation-aspiration instrument) to break up a cataract into small fragments and emulsify (liquefy) them, making them easier to suction out of the eye.


Phakic (FAY-kik)
Refers to an eye that possesses its natural lens.


Phoropter (for-AHP-tur), refractor
Refraction device incorporating a series of spherical and cylindrical lenses, with prisms, occluders and pinholes. For determining an eyes optical correction.


Photodynamic Therapy (PDT)
Two-step technique for treating neovascularization (abnormal new blood vessels) associated with age-related macular degeneration (AMD). First, a dye that concentrates in neovascular blood vessels is injected into an arm vein. A few minutes later, those blood vessels are treated with a laser to close them off. Treatment often needs to be repeated.


Photophobia (foh-toh-FOH-bee-uh)
Abnormal sensitivity to, and discomfort from, light. May be associated with excessive tearing. Often due to inflammation of the iris (iritis) or cornea (keratitis).


Phototherapeutic keratectomy (PTK), therapeutic photokeratectomy
Use of an excimer laser to remove corneal tissue, smooth an irregular corneal surface, or remove superficial corneal scars.


Pinhole (ph)
Opaque disc with one or more holes ranging from 0.5 to 2 mm in diameter. Looking through the hole with one eye (other eye covered) will improve vision if reduced vision is caused by an optical defect or refractive error.


Pink eye


Plano (PLAY-noh)
Lens that has no focusing power, neither plus nor minus.


Separation of light into beams that vibrate in one plane, instead of in all planes as does ordinary light.


Transparent plastic polymer used for making safe spectacles. Characterized by extreme resistance to breaking or shattering.


Measure (in diopters) of the capability of a lens to converge (plus lens) or diverge (minus lens) light rays.


Diminished power of accommodation arising from loss of elasticity of the crystalline lens and/or loss of ciliary muscle function, as occurs with aging; usually becomes significant after age 45. May also be caused by diseases such as hypothyroidism or glaucoma.


Wedge-shaped, transparent medium that bends light rays toward its base. Does not focus.


PRK Photorefractive keratectomy
Use of a computer-controlled excimer laser to reshape the corneal curvature (changing its optical power) after the surface layer of the cornea (epithelium) is removed by gentle scraping. Corrects myopia, hyperopia, and astigmatism.


Progressive addition lens (PAL), invisible bifocals
Eyeglass lens that incorporates corrections for distance vision, through midrange, to near vision (usually in lower part of lens), with smooth transitions and no bifocal demarcation line.


Prosthesis (prahs-THEE-sis), shell
Cosmetic false eye replacement for a removed (enucleated) eye. Plexiglas shell painted to resemble a natural eye; fits into conjunctival sac under the eyelids and over a buried implant.


Pseudophakia (SU-soh-FAY-kee-uh)
State of having an intraocular lens implant that replaces the eye's natural lens.


Pterygium (tur-IH-jee-um)
Abnormal wedge-shaped growth on the bulbar conjunctiva. May gradually advance onto the cornea and require surgical removal. Probably related to sun irritation.


Ptosis (TOH-sis), blepharoptosis
Drooping or sagging or the upper eye lid.


Punctual plug
Plastic material (polyhydroxethyl methacrylate or silicone) inserted into the punctum to prevent normal tear drainage; to preserve tears for helping keep the cornea and conjunctiva moist.


Punctum (lacrimal)
Tiny skin opening of each upper (superior) and lower (inferior) eyelid, near the nose. Entrance to the tear drainage (lacrimal) system. Each punctum is surrounded by a small papilla (elevation) of skin.


Variable-sized black circular opening in the center of the iris that regulates the amount of light that enters the eye.


1. Instrument used for measuring the distance from the center of the pupil to the center of the bridge of the nose (monocular interpupillary distance), for fitting spectacles. 2. Laboratory instrument or measuring size (diameter) of the pupil.


Radial Keratotomy (keh-ruhTAH-tuh-mee)
See RK.


Red eye
Lay term for any condition (e.g. conjunctivitis, uveitis) with dilation of conjunctival or ciliary blood vessels.


1. Bending of light rays as they travel from a clear medium of one density to another of different density. 2. Determination of an eye's refractive error and the best corrective lenses to be prescribed; series of lenses in graded powers are presented to determine which provide sharpest, clearest vision. 3. Prescription for eyeglasses or contact lenses resulting from this test.


Refractive error
Optical defect in an unaccommodating eye; parallel light rays are not brought to a sharp focus on the retina, producing a blurred retinal image. Can be corrected by eyeglasses, contact lenses, or refractive surgery.


Refractive Lens Exchange (RLE), lens extraction, clear lens extraction (CLE)
Removal of the eye's natural lens to correct high myopia or hyperopia. An intraocular lens or multifocal IOL may be implanted to supplement the refractive correction.


Refractive surgery
Any surgical procedure to change the eye's refractive error. Can reduce or eliminate the need for eyeglass or contact lens correction.




Slow loss of refractive correction following photorefractive surgery. A reason for intentional surgical overcorrection.


Relaxing incisions


Retina (RET-ih-nuh)
Light sensitive nerve tissue in the eye (embryologically part of the brain) that converts images from the eye's optical system into electrical impulses that are sent along the optic nerve to the brain, to interpret as vision. Forms a thin membranous lining of the rear two-thirds of the globe; consists of layers that include rods and cones; bipolar, amacrine, ganglion, horizontal and Muller cells; and all interconnecting nerve fibers.


Retinal detachment (RD), retinal separation
Separation of the retina from the underlying pigment epithelium. Almost always caused by a leaking retinal tear, which allows fluid to pass from the vitreous into the sub-retinal space. Disrupts visual cell structure and thus markedly disturbs vision; often requires immediate surgical repair.


Retinal hole retinal tear, retinal break
Tear through retinal tissue. Usually caused by a tug or traction by the vitreous; may leak, causing a retinal detachment.


Retinitis pigmentosa (RP)
Progressive retinal degeneration in both eyes. Night blindness, usually in childhood, is followed by loss of peripheral vision (initially as ring-shaped defect), progressing over many years to tunnel vision and finally blindness. Hereditary.


Retinopathy (ret-in-AHP-uh-thee)
Any non-inflammatory degenerative disease of the retina.


RK (radial keratotomy)
Method of flattening the central cornea with 4-8 spoke-like (radial) incisions in the corneal periphery, reducing the cornea€™s optical power. Corrects myopia.


See Refractive Lens Exchange.


Rosacea (roh-ZAY-shuh)
Skin disease accompanied by chronic staphylococcal infections of the cheeks and eyelids (blepharitis) and, rarely, corneal, sclera or iris inflammations. Cause unknown.


Safety lens
Eyeglass lens treated (with heat, chemicals, or lamination) to resist breakage and splintering from a direct blow.



Sclera (SKLEH-ruh)
Opaque, fibrous, protective outer layer of the eye (€œwhite of the eye€) that is directly continuous with the cornea in front and with the sheath covering the optic nerve behind. Contains collagen and elastic fibers.


Scleral buckle (SKLEH-rul)
Technique used in repairing a retinal detachment; material (usually silicone rubber) is sutured onto the sclera to indent (or buckle) it inward, applying localized pressure over the retina, to help seal a tear or reduce vitreous traction.


Snellen chart
For assessing visual acuity. Contains rows of letters, numbers, or symbols in standardized graded sizes, with a designated distance at which each row should be legible to a normal eye. Usually tested at 20 feet (6 m).


Soft contact lens (SCL)
Hydrophilic (water-absorbing) small plastic disc; for correcting a refractive error or protecting a damaged corneal surface. Rests on the cornea; more comfortable than a hard contact lens.




Lens whose optical power (in diopters) is the same in all meridians.


Spherical equivalent (SE)
Average power of a toric lens, equal to the sum of the spherical power plus half the cylindrical power. Represented by the dioptric power of a spherical lens that positions the circle of least confusion.


Perception of bright lines radiating from a small light source, such as car headlights. Caused by any opacity in the eye's optical system, e.g. corneal swelling, cataract, or even smeared spectacle lenses.


Strabismus (struh-BIZ-mus)
Eye misalignment or eyes that do not move normally, caused by extraocular muscle imbalance. One fovea is not directed at the same object as the other.


Striae (STREE-uh)
In the choroid or cornea, wrinkles or folds that may be caused by abnormal pressure or distortion of tissue.


Framework, usually of connective tissue or an organ, gland or other structure.


Stye sty
Acute pustular infection of the oil glands of Zeis, located in an eyelash follicle at the eyelid margin.


Subconjunctival hemorrhage (sub-kahn-junk-TI-vul HEM-uh-rij)
Bleeding from a small blood vessel under the conjunctiva; often spontaneous or from coughing. Seen as a bright-red blood over the sclera. Harmless; blood absorbs in about one week without treatment.


Sunglasses absorptive lenses
Spectacles whose lenses absorb a high percentage of light, thus reducing the amount of light transmitted to the eye. Worn in bright sunlight for comfort and for protection from light and ultraviolet (UV) damage.


Subjective, personal sensation relating to a physical condition or disturbance.


Group of signs or symptoms that tend to occur together and characterize a particular abnormality.


Tear drainage system lacrimal apparatus
Orbital structures for tear production and drainage. Tears (produced in lacrimal gland above eyeball) flow across corneal surface, drain into upper and lower puncta (openings at inner eyelid margins), through upper and lower canaliculi to common canaliculus, into the tear sac, then through the nasolacrimal duct into the nose.


Tear duct lacrimal duct
Tear drainage channel that extends from the lacrimal sac to an opening in the mucous membrane of the nose.


Tear film
Liquid that bathes the cornea and conjunctiva. Three layers; outer oily layer secreted by the meibomian glands, middle aqueous layer secreted by the lacrimal glands, inner mucin layer produced by the conjunctival goblet cells.


Fluid secreted by lacrimal glands; keep the conjunctiva and cornea moist.


Tonometer (tuh-NAH-mih-tur)
Device that measures intraocular pressure.


Refers to local application to, or action on, the surface of a body part.


Process of reading a map that describes the shapes of surfaces. Shows relative elevations connected by contour lines. Corneal: map of the variations in front surface curvature of the cornea.


Toric lens (TOR-ik)
Lens that has a cylindrical component; used for correcting an astigmatic refractive error. Most eyeglasses are of this type.


Trifocal (TRI_foh-kul)
Eyeglass lens that incorporates three lenses of different powers. The main portion is usually focused for distance (20 ft.), the center segment for about 2 ft., and the lower segment for near (14 in.).


Tunnel vision
Loss of peripheral visual field with retention of some central field. Typical of the end stage of glaucoma and retinitis pigmentosa.


Normal visual acuity. Upper number is the standard distance (20 ft) between an eye being tested and the eye chart; lower number indicates that the tested eye can see the same small standard-sized letters or symbols as a normal eye at 20 feet.


Ultraviolet (UV)
Light with wavelengths between about 250-400 nm (invisible to naked eye.)


Uncorrected visual acuity (UCVA)
Vision obtained without using an optical aid.


Any correction, surgical or optical, that is less than complete.


Uvea (YU-vee-uh)
Pigmented vascular layers of the eye (iris, ciliary body, choroid); contain most of the intraocular blood vessels.


Ability of the eye to receive, resolve and transmit light images to the occipital lobe in the brain, where the light sensation is interpreted. Seeing ability in its broadest sense.


Visual acuity


Visual field
Full extent of the area visible to an eye that is fixating straight ahead. Measured in degrees from fixation.


Vitrectomy (vih-TREK-tuh-mee)
Removal of vitreous, blood, and/or membranes from the eye, with a needle-like cutting device that has suction and fluid injection capabilities.


Refers to conditions, diseases, or treatments that involve both the vitreous and the retina.


Vitreous (VIT-ree-us), vitreous body, vitreous gel, vitreous humor Transparent, colorless gelatinous mass (fine collagen fibrils and hyaluronic acid) that fills rear two-thirds of the eyeball, between the lens and the retina.


Vitreous floaters
Particles that float in the vitreous and cast shadows on the retina; seen as spots, cobwebs, spiders, etc. Occur normally with aging or in association with vitreous detachment, retinal tear, or in intraocular inflammation.


Wavefront analyzer aberrometer
Device that permits study of the eye's image-forming quality and optical aberrations, and how they affect visual acuity. Aids laser-controlled refractive surgery (PRK, LASIK) to create a more accurate optical correction.


White of the eye


YAG laser
Laser that produces short pulsed, high energy light beam to cut, perforate, or fragment tissue. Acronym: Yttrium-Aluminum-Garnet.


Zernike polynomial
Analytic method for measuring and describing optical aberrations. Useful for laser-controlled corneal shaping.

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BayCare Clinic,, is the largest physician-owned specialty-care clinic in northeastern Wisconsin and Michigan’s Upper Peninsula. It is based in Green Bay, Wisconsin. BayCare Clinic offers expertise in more than 20 specialties, with more than 100 physicians serving in 16 area communities. BayCare Clinic is a joint partner in Aurora BayCare Medical Center, a 167-bed, full-service hospital. Follow BayCare Clinic on Facebook and Twitter.