The A to Z of Eye Terms Commonly Used By Eye Care Practitioners

This post aims to introduce some of the wide variety of eye-related terms and issues you may encounter, and what they mean in common terms.

Please note, the content included here is not intended to be used for clinical or diagnostic purposes.


Adie’s Pupil – A neurological disorder affecting the pupil of the eye. The affected eye appears more dilated than the normal eye.

Adult Strabismus – A form of visual disturbance caused by the misalignment of eyes.

Age–related Macular Degeneration – Age related, deterioration of the macula

Amblyopia – Decreased visual acuity in one or both eyes, usually due to lack of sensory input during childhood. Often occurs without any apparent disease of the eye.

Aqueous Humor – Clear, watery fluid which fills the anterior chamber of the eye.

Astigmatism – A refractive error in which light rays fail to come to a single focus on the retina because of differing amounts of refraction in the various meridians of the eye.

Anterior Chamber – The anterior chamber is the front part of the eye between the cornea and the iris.


Blepharitis – A refractive error in which light rays fail to come to a single focus on the retina because of differing amounts of refraction in the various meridians of the eye.

Blocked Tear Duct – The condition is caused by a partial or complete obstruction in the tear drainage system.

Branch Retinal Vein Occlusion (BRVO) – An eye condition in which a blood clot stops or slows down blood circulation in a branch of the large vein within the retinal tissue.


Cataract – A clouding of the crystalline lens of the eye or its capsule, or both, with consequent loss of visual acuity

Central Retinal Vein Occlusion – An eye condition in which a blood clot stops or slows down blood circulation in the central retinal vein.

Central Serous Chorioretinopathy – A condition that causes central vision loss due to fluid leaking into the retina, particularly the macula.

Chalazion – Inflammatory enlargement of a meibomian gland in the eyelid.

Choroid – The vascular, intermediate layer of the eye, lying between the retina and sclera, which furnishes nourishment to the other parts of the eyeball.

Color Deficiency – Diminished ability to perceive differences in colour – usually for red or green, rarely for blue or yellow.

Cones –  A type of photoreceptor cell in the retina that are responsible for the visual acuity of the eye.

Conjunctiva – Mucous membrane which lines the eyelids and covers the front part of the eyeball.

Conjunctivitis – Inflammation of the conjunctiva.

Cornea – The transparent, dome-shaped tissue covering the iris and the pupil.

Corneal Abrasion – An erosion of the superficial layer of the epithelium of the cornea (missing or injured cells); normally quite painful.

Contrast Sensitivity – A measurement which determines the observer’s ability to see a wide range of everyday objects under normal and reduced illumination conditions.

Cup–to–disc ratio (C/D) – the relationship between the size of the optic cup and the optic disc of the optic nerve.

Cycloplegic – A drug that temporarily paralyzes the ciliary muscle and dilates the pupil; often used to ascertain the refractive error.

Cylindrical Lens – A toric lens that has differing radii of curvature in different meridians, which causes the refractive power to vary in the different meridians; it is used to correct astigmatism.


Dark spots or curtains in vision – Visual changes that include seeing dark spots ordark curtain covering part of your side vision.

Dacryocystitis – Inflammation of the lacrimal sac.

Dark Adaptation – The ability of the eye to adjust to a dimly lit environment.

Decentration – The displacement of the optical center of an eyeglass lens.

Depth Perception – The visual ability to perceive the distance of objects and the spatial relationship of objects at different distances in three dimensions.

Diopter – A unit of measurement of strength or refractive power of lenses; it can also refer to therelative curvature of a lens surface.

Dilated Pupil – A condition that results when the opening in the center of the iris grows bigger to let in more light.

Diplopia – Double vision

Drainage Angle – The area of the eye where the aqueous humor drains from the front of the eye

Drusen – A collection of small extracellular deposits of lipids and proteins that appear in one of the layers of the retina in the eye.

Dry Eyes – A common condition that occurs when the eyes do not produce enough tears, or the tears evaporate too quickly.

Dyslexia – A dysfunction in which the ability to read is diminished due to neurological or othernon–optical problems.


Emmetropia – The focal condition of the normal eye in which there is no refractive error.

Endophthalmitis – Inflammation of most of the internal tissues of the eyeball.

Entropion – A condition in whichthe eyelid folds inward, towards the eye.

Enucleation – Complete surgical removal of the eyeball.

Esophoria – A tendency of the eyes to turn too far inward under certain conditions.

Esotropia – A condition in which one or both eyes turn too far inward at all times; sometimes called convergent strabismus or crossed eyes.

Exophoria – A tendency of the eye to turn too far outward under certain conditions.

Exophthalmos – An abnormal protrusion of the eyeball.

Exotropia – A condition in which one or both eyes turn too far outward at all times; sometimes called divergent strabismus.

Extraocular/ Extrinsic Muscles – Six muscles that are responsible for moving the eye. Each eye has four recti and two oblique muscles.

Eye Dominance – The tendency of one eye to assume the major function of seeing, being assisted by the less dominant eye.


Far–sightedness – See “Hyperopia.”

Field of Vision – The entire area which can be seen without shifting the gaze.

Fixate – The ability of an eye to move so an image is focused on the fovea.

Floaters – Small particles consisting of cells or fibrin which move in the vitreous.

Fornix – The pocket formed where the bulbar conjunctiva meets the palpebral conjunctiva.

Fovea – A small depression in the central retina at the back of the eye; the part of the macula adapted for most acute vision.

Fresnel prism – A series of small prisms in a plastic sheet, which attach to eyeglass lenses to temporarily correct for eye misalignments.

Fundus – The back of the eye including the retina, optic nerve, macula, and fovea.

Fusion – The power of coordinating the images received by the two eyes into a single image.


Gas Permeable Lenses – Contact lenses that allow oxygen and carbon dioxide to pass through them. This usually refers to a rigid lens, although soft lenses are also gas permeable.

Glare – Caused by a brightness greater than the eyes can adjust to.

Glaucoma – A disease in which vision is lost due to damage of the optic nerve, it is often associated with increased pressure inside the eye.

Gonioscope – A magnifying device, which is used to examine the angle of the anterior chamber.

Giant papillary conjunctivitis (GPC) – A type of hypersensitivity or allergic reaction, resulting in bumps called papillae developing on the underside of the upper eyelid, usually accompanied by mucus discharge and itching.


Hemianopsia – Blindness of one half the field of vision of one or both eyes.

Hyperopia – Often called farsightedness. It is a refractive condition resulting from the tendency of rays of light to be out of focus on the retina when accommodation is relaxed.

Hypophoria – A tendency of one eye to deviate downward under certain conditions.

Hypotropia – A condition in which one eye deviates downward all the time.


Injection – A term used sometimes to mean congestion of ciliary or conjunctival blood vessels. redness of the eye.

Interpupillary Distance – Also called pupillary distance (PD), is the distance between the centers of the pupils of each eye (measured in millimeters).

Intraocular Lens (IOL) – An artificial lens that is implanted in the eye during cataract surgery to replace the anatomical lens.

Intraocular Pressure (IOP) – The fluid pressure or tension within the eye measured in millimeters of mercury (mmHg)

Iris – The colored, circular muscle tissue behind the cornea and immediately in front of the crystalline lens. The iris controls the amount of light entering the eye by changing the size of the pupil.

Iritis – Inflammation of the iris; the condition is marked by pain, inflammation, discomfort from light, contraction of pupil, discoloration of iris.


Jaeger Test – A test for near vision; this includes lines of reading matter printed in a series of various sizes of typeface.


K–readings – Corneal curvature measurements taken with a keratometer

Keratitis – An inflammation of the cornea.

Keratoconus – A progressive disease where the central corneal tissue protrudes forward dramatically, making the cornea somewhat cone shaped.

Keratometry – The measurement of the anterior curvatures of the cornea with a keratometer.

Keratoplasty – The graft of a donor cornea to replace a damaged or diseased cornea.


Lacrimal Gland – A gland in the eye which secretes tears.

Lacrimation – Production of tears.

LASIK – Lasik stands for “Laser-assisted in situ keratomileusis” which is commonly referred to as laser vision correction or laser eye surgery, which is a type of corneal refractive surgery for the correction for myopia, hyperopia, and astigmatism.

Lateral – Toward the ear, away from the midline of the body.

Lazy eye/Amblyopia – A condition which causes decreased visual acuity in one or both eyes with no detectable cause.

Legal Blindness – Having central visual acuity of 20/200 or less in the better eye after correction or having visual acuity of better than 20/200 but having a visual field of no greater than 20 degrees in the better eye.

Lens – A refractive medium having one or both surfaces curved.

Light Adaptation – The ability of the eye to adjust to an increased amount of light in the environment.

Limbus – The boundary between the cornea and sclera.

Low Vision Aids – Optical devices of various types useful to persons with vision impairment.


Macula – The small area of the central retina that surrounds the fovea; this region provides the most distinct vision in the retina.

Meibomian Gland – A gland located within the eyelids which secretes an oily layer of tear fluid.

Medial – Toward the nose or toward the midline of the body.

Miotic – A drug that causes the pupil to contract.

Monocular – Refers to one eye only.

Monovision – A type of contact lens fitting in which one eye is corrected for distance vision and the other is corrected for near vision.

Mydriatic – A drug that dilates the pupil.

Multifocal Lens – Eyeglass lens that incorporates several different optical powers and allows the wearer to see clearly at multiple distances.

Myopia – Often called nearsightedness, Myopia is a refractive error in which because the eyeball is too long in relation to its focusing power, the focal point for rays of light from distant objects is in front of the retina.


Near Point of Accommodation – The nearest point at which the eye can perceive an object distinctly. It varies according to the power of accommodation.

Near Point of Convergence – The nearest single point at which the two eyes can direct their visual lines, normally about three inches from the eyes in young people.

Near Vision – The ability to perceive distinctly objects at normal reading distance, or about fourteen to sixteen inches from the eyes.

Nearsightedness – See “Myopia.”

Neovascularization – Formation of new blood vessels in a part of the eye. Can occur in the cornea when it does not receive enough oxygen.

Night Blindness – A condition in which vision diminishes greatly in faint light.

Nystagmus – An involuntary, rapid movement of the eyeball; it may be lateral, vertical, or rotary.


Occluder – An opaque or translucent device that is placed before an eye to block vision.

Oculus Dexter (O.D.) – Right eye

Oculus Sinister (O.S.) – Left eye

Oculi Uniter (O.U.) – Both eyes

Ophthalmologist – A medical doctor who specializes in diagnosis and treatment of defects and diseases of the eye, performing surgery when necessary or prescribing other types of treatment, formerly called an oculist.

Ophthalmoscope – An instrument used to examine the interior of the eye.

Optic Atrophy – Degeneration of the nerve tissue, which carries messages from the retina to the brain.

Optic Disk – Head of the optic nerve in the eyeball where the retinal nerve fibers leave the eye, responsible for the physiological blind spot.

Optician – One who dispenses frames and lenses, based on a prescription written by an

Optometrist or Ophthalmologist – One who grinds lenses, fits them into frames, and adjusts the frame to the wearer.

Optic Nerve – The nerve which carries visual messages from the retina to the brain.

Optic Neuritis – Inflammation of the optic nerve.

Optometrist – A primary healthcare provider specializing in diagnosing and treating diseases and optical disorders of the eye, prescribing lenses, vision training, medicines, and other treatments.

Orthokeratology – The practice of purposely flattening the cornea with a specially designed contact lens. Primarily used to reduce or eliminate myopia.

Orthoptic Training – series of scientifically planned exercises for developing or restoring the normal teamwork of the eyes.


Palpebral – Pertaining to the eyelid.

Pantoscopic Tilt – The angle of an eyeglass lens in relation to the vertical plane.

Perimeter – An instrument for measuring the field of vision.

Peripheral Vision – The ability to perceive the presence, motion, or color of objects outside of the direct line of vision.

Phakic – Refers to an eye that still possesses its natural lens.

Phoria – A latent deviation in which the eyes tend to turn from the normal position for binocular vision.

Phoropter – An instrument for determining the refractive and binocular state of the eye.

Photophobia – Abnormal sensitivity to and discomfort from light.

Pinguecula – A triangular patch of thickened tissue on the bulbar conjunctiva on either side of the cornea.

Posterior Chamber – The narrow space between the back of the iris and the front surface of the crystalline lens bounded by the ciliary body and filled with aqueous.

Prentice’s Rule – Formula for calculating the prismatic effect induced at any point in a lens, prism diopters equal decentration (in cm) times lens power in diopters.

Presbyopia – A condition common after age forty in which a gradual lessening of the eye’s power of accommodation causes difficulty in seeing near objects.

Prism – A triangular, wedge shaped lens that bends, but does not focus, light rays toward the position of its base.

Prism Ballast – When a contact lens is thicker and heavier at the bottom, so that it will remain oriented in a specific position.

Progressive Addition Lens – Also referred to as a “PAL”, a lens that gradually increases in plus power towards the bottom of the lens.

Prosthesis – An artificial substitute for a missing eye (or other missing part of the body).

Pseudoisochromatic Charts – Charts with colors dots of various hues and shades indicating numbers, letters, or patterns, used for testing color discrimination.

Pterygium – A triangular fold of growing membrane which invades the cornea, usually on the medial side.

Ptosis – A paralytic drooping of the upper eyelid.

Pupil – The circular opening in the center of the iris which regulates the amount of light that enters the eye.


Refraction – In optics, refraction is the bending of light rays as they travel from one medium to another. Also, a testto determine the refractive error of an eye and the best corrective lenses to be prescribed.

Refractive Error – A defect in the visual system of the eye that prevents light rays from being brought to a single focus exactly on the retina.

Retina – A thin membrane lining the back of the eye where optical images are received. The retina converts these images into electronic impulses and transmits them to the brain along the optic nerve.

Retinal Detachment – A separation of the retina from the choroid.

Retinitis – An inflammation of the retina.

Retinitis Pigmentosa – A hereditary degeneration and atrophy of the retina, which is usually accompanied by misplaced pigment in the retina.

Retinopathy – A disease of the retina, due to various causes.

Retinoscope – An instrument for determining the refractive state of the eye by observing the movements of lights and shadows across the pupil by the light thrown onto the retina from a moving mirror.

Rods and Cones – These are two types of nerve cells which form a layer of the retina and act as light receiving media. Cones are concerned with visual acuity and color discrimination, rods with motion and vision at low degrees of illumination (night vision).


Safety Glasses – Impact–resistant spectacles, which are available with or without visual correction for protection of the eyes in hazardous situations.

Sclera – The white part of the eye – a tough covering which, with the cornea, forms the external protective coat of the eye.

Scleritis – Inflammation of the sclera.

Scotoma – A blind or partially blind area in the visual field.

Segment – Also referred to as “seg”, an area of a lens with power differing from that of the main portion.

Snellen Chart – Used for testing distance visual acuity. It consists of lines of letters, numbers or symbols in graded sizes drawn to Snellen measurements. Each size is labeled with the distance at which it can be read by the normal eye. Most often used for testing vision at 20 feet.

Soft Lens – A contact lens made of soft plastic that contains water and allows for oxygen transmission. Soft lenses are characterized by comfort and ease of adaptation.

Spherical Lens – A lens, which refracts rays of light equally in all meridians.

Stereoscopic Vision/Depth Perception – The ability to use both eyes together to perceive the relative position of objects in space.

Strabismus/Tropia/Squint – This is a failure of the two eyes simultaneously to direct their gaze at the same object because of muscle imbalance.

Stye – Acute inflammation of a sebaceous gland in the margin of the eyelid, due to an infection and usually resulting in the formation of pus.


Telescopic Glasses – Magnifying spectacles founded on the principles of a telescope; occasionally prescribed for improving very poor vision which cannot be improved by ordinary glasses.

Tonometer – An instrument for measuring pressure inside of the eye.

Toric – Lens with two different power curves at defined optical axes, used to correct astigmatism.

Transpose – To convert the prescription of a sphero-cylindrical lens into the oppositely poweredcylinder form (a plus cylinders into minus, or vice versa).

Trifocal – Lenses that have three areas of viewing, each with its own power.

Truncation – A flattening of the lower edge of a contact lens, so that it will remain oriented in aspecific position.

Tunnel Vision – Contraction of the visual field to such an extent that only a small area of centralvisual acuity remains, thus giving the affected individual the impression of looking through atunnel.


Uveal Tract – The entire vascular layer of the eyeball, including the iris, ciliary body, and choroid.


Vertex Distance – The distance from the front surface of the eye to the back surface of thespectacle lens.

Visual Acuity – The resolving power of the eye, or the ability to see two separate objects asseparate. Ability of the eye to perceive the shape of objects in the direct line of vision, usuallymeasured in terms of a Snellen fraction, e.g. 20/20.

Vitreous – The gelatinous transparent, colorless substance filling the space in the eyeball betweenthe crystalline lens and the retina.


Zonules – Radially arranged fibers that suspend the lens from the ciliary body and hold it in.

Give us a call to learn more about any of the above eye terminologies or make an appointment for free eye test with an optometrist at a MAGRABi store near you.

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