For Little Eyes Glossary

Image of a book with "For Little Eyes glossary" written on the pages

I’ve tried to compile easy-to-understand definitions of the most common terms and phrases that have to do with children’s vision.  I’ve also included links to more information or to posts on this site about the topic when appropriate.  Please let me know if there are other terms you would like to see added!

a b c d e f g h i j k l m n o p q r s t u v w x y z



Accommodation is the ability the focus on things close up or to focus through hyperopia. The accommodative reflex is closely tied to the muscles that turn the eyes inward, so excessive use of accommodation can lead to esotropia.


Acuity is how clearly you see. Usually this is measured by having someone read letters or pictures from an eye chart. It is expressed as two numbers, ex: 20/20 or 6/6. The first number tells you the distance that the acuity test was done at: 20 is in feet, 6 is in meters. The second number indicates how far away a person with normal acuity would be in order to see things as clearly as you do. So 20/40 means that what you see clearly at 20 feet, a person with normal acuity could see at 40 feet. When both numbers are the same that means the acuity is normal. If the second number is smaller, then the acuity is better than normal. If the second number is larger, then the acuity is worse than normal.  


On prescriptions for bifocals, the add tells you the additional prescription to add to the bottom part of the lens.


Referring to a lack of pigment in the skin, hair, and eyes. This can cause many vision issues. See ocular albinism


Sometimes known as “lazy eye”.  Amblyopia is when the brain has suppressed the visual input from one or (less commonly) both eyes. This leads to reduced visual acuity despite there being no mechanical reason for poor vision (a person with amblyopia might wear glasses to correct their refractive error, but still not see clearly). This can happen for a few reasons:

  • anisometropic amblyopia: when the refractive error in one eye is significantly worse than the other so the brain has suppressed the vision in the worse eye.
  • deprivation amblyopia: when the vision from one eye is blocked so the brain suppresses the vision from the blocked eye. Most commonly this happens when there is a cataract in one eye.
  • refractive amblyopia: when there is a significant refractive error in one or both eyes and the brain suppresses the vision in the eye(s).
  • strabismic amblyopia: when the eyes are not aligned and the brain suppresses the vision from one eye to eliminate double vision.

Typically, amblyopia is treated by first resolving any vision issues (for instance, prescribing glasses if needed, or removing a cataract). After that, the non-amblyopic (better seeing eye) is often patched or penalized (so it cannot see as clearly) to encourage the brain to re-develop visual connections of the amblyopic eye.  Another treatment is vision therapy which encourages the eyes to work together.

Read posts about amblyopia.


When the iris (the colored part of the eye around the pupil) is missing.

Read posts about aniridia.


When the refractive error for each eye is different. Untreated anisometropia can lead to amblyopia.


When one eye has hyperopia (farsightedness) and the other has myopia (nearsightedness).


When the lens of the eye has been surgically removed, usually due to cataracts.


A refractive error that happens when the cornea is not perfectly round, but rather oval shaped (think the shape of an American football or a rugby ball). This is called the cylindrical error. This results in the prescription not being the same in all parts of the eye.

The amount of astigmatism is usually written in a prescription as “CYL”. The cylindrical error number indicates the greatest difference from the base prescription for that eye. Most ophthalmologists will write the astigmatism with a +, most optometrists write it with a -.  The second part of the astigmatism prescription is the axis, sometimes written as “X”.  This describes the orientation of the cylinder prescription.  It will be a number between 1 and 180 and will not have anything to do with the severity of the astigmatism.

Read more about eye glasses prescriptions.


A medication used as an eye drop. It is a cyclopegic, which means it dilates the eye and stops the accommodative reflex. Atropine is sometimes used in the treatment of amblyopia, by putting drops in the non-amblyopic eye, which blurs the vision in that eye. Atropine has also shown promise in slowing the progression of myopia in children.


Part of an eye glasses prescription for astigmatism. The axis indicates which way the astigmatism prescription is situated on the eye. It is expressed in degrees between 0 and 180. That angle is the axis along which the cylindrical error is applied to the base prescription. Ninety degrees from that axis is where there is no cylindrical error. On a prescription, it is usually written as “AXIS”: or “X”.

Read more about eye glasses prescriptions.



A bifocal lens is one in which there is a stronger hyperopic prescription at the bottom part of the lens.  This allows the person wearing the glasses to get a stronger prescription when they look down at closer objects. There are many different types of bifocal lenses depending on how the lens is cut.  See also: glasses prescription


Bilateral is used to describe a condition that affects both eyes. For example, bilateral cataracts refer to cataracts that are in both eyes.


The condition of not having useful sight. Blindness may be temporary or permanent and can be due to many different causes.

See also legally blind


The part of the glasses frame that goes over the nose between the two lenses. Also called the nosepiece.

drawing of glasses with parts labeled: lenses, bridge (nose piece), and temples (ear pieces)



A condition in which the lens of the eye becomes cloudy. Cataracts can be congenital, which means they were there from birth, or they can be acquired. Cataracts are usually treated with surgery to remove the lens.

Read posts about cataracts.


An infection of the conjunctiva (the white of the eye). This can be viral or bacterial. Often referred to as “pink eye”.


Removable lenses that set on top of the eye. They usually provide correction for refractive errors (myopia, hyperopia, or astigmatism).


Certified Optometric Vision Therapist:  Credentials that indicate that a vision therapist has undergone additional training and certification to provide vision therapy with a board certified optometrist FCOVD.

cortical visual impairment (CVI):

A condition in which the eyes may be functioning correctly, however, the brain is not processing the visual inputs.

Read posts about cortical visual impairment.

cylinder or cylindrical error:

see astigmatism.



A part of most eye exams, dilation is a way of temporarily paralyzing the muscles in the eye so that the pupil stays dilated. This allows eye doctors to view the back of the eye during an exam, it also keeps the eye from accommodating (a reflex to focus through farsightedness), so that the eye doctor can get a sense of the true amount of refractive error. When the eye is dilated, it will not be able to focus up close and will be sensitive to bright lights.


The unit of measurement for the sphere and cylinder component of a glasses prescription. This describes the strength of the lens. The higher the diopter number, the stronger a prescription is.


A condition in which a person is seeing double. Usually, this is due to the eyes not being aligned, so the brain is getting a different image from each eye and cannot combine them. 


ectopia lentis:

A vision disorder in which the eye’s lens is out of its normal position. Ectopia lentis is often associated with other conditions such as Marfan syndrome, or it may not have any identifiable cause, in which case it is referred to as “idiopathic ectopia lentis.” The treatment is often to remove the lens, leading to aphakia.

Read posts about idiopathic ectopia lentis.


When there is no refractive error (no hyperopia, no myopia, and no astigmatism)


A condition in which the eyes turn inward or cross. Also known as crossed eyes. Esotropia is a form of strabismus.

exam under anesthesia (EUA):

An eye exam that is done while the patient is under general anesthesia. This is often done in young children at risk of glaucoma to determine their intraocular pressure.


A condition in which the eyes turn outward. Sometimes called wall-eyed. Exotropia is a form of strabismus.



Having hyperopia. Also known as longsighted.  See also: glasses prescription.


Fellow of the College of Optometrists in Vision Development. An optometrist who has undergone additional training and Board Certification in Vision Development and Vision Therapy.


glasses prescription:

The prescription that describes how lenses should be cut for a pair of glasses. Parts of the prescription include:

  • Sphere (SPH): the amount of correction for myopia (nearsightedness or shortsightedness) or hyperopia (farsightedness or shortsightedness)
  • Cylinder (CYL): the amount of correction for astigmatism
  • axis (X) : the position of the astimatism
  • The prescription may also include:
    • add — on prescriptions for bifocals
    • prism — if a prism is needed on the

Read more about how to read a glasses prescription.


A medical condition in which high intraocular (eye) pressure causes damage to the optic nerve. Primary glaucoma is glaucoma that has no identifiable cause. Glaucoma can also be secondary, if it is associated with another cause, such as cataract surgery. Pediatric glaucoma can be categorized as follows:

  • congenital glaucoma: glaucoma that has existed from birth
  • infantile glaucoma: glaucoma that develops after birth, but before the age of 3 years old
  • juvenile glaucoma: glaucoma that develops after age 3, but before 18
  • primary glaucoma: glaucoma with no identifiable underlying cause
  • secondary glaucoma: glaucoma that is caused by or associated with another condition

Read posts about glaucoma.



A glasses lens material that bends light more than traditional lens materials. Because of this, high index lenses can correct a prescription with a thinner lens than other lens materials. High index lenses are particularly helpful in high prescriptions.


Also known as farsightedness or longsightedness. Hyperopia is a type of spherical error in which the eye is too short so light is focused behind the retina. This causes close-up images to be blurry, while far away images might be clearer. In a glasses prescription, hyperopia is indicated with a “+” in front of the spherical error.  See also: glasses prescription

Read posts about hyperopia.


intraocular pressure (IOP):

The pressure inside the eye. High intraocular pressure can damage the ocular nerve and lead to glaucoma.

intraocular lens (IOL):

A surgically implanted lens. Intraocular lenses are often implanted after cataract surgery removes the lens.


lazy eye:

A non-medical term that can have a number of different meanings that often leads to confusion. It is better to use one of the more specific terms to refer to the vision issue. Possible meanings include:

  • amblyopia: when one or both eyes are not seeing as clearly as expected despite correction from glasses or contacts.
  • strabismus: when the eyes are not aligned (they either cross or point out)
  • ptosis: when the eyelid of one or both eyes droops down covering part of the eye.

legally blind:

This term is often used to refer to visual acuity of less than 20/200, however, the legal definition of blindness is much more complicated. “Legal blindness” is a definition used by the United States government to determine eligibility for vocational training, rehabilitation, schooling, disability benefits, low vision devices, and tax exemption programs. The legal US definition has 2 parts:

  • visual acuity of 20/200 (6/60) or less in the better-seeing eye while wearing glasses or contacts.

~ OR ~

  • a visual field of 20 degrees or less (this is known as tunnel-vision) in the better-seeing eye.

Read the official US government definition.


Another name for having hyperopia, also called “farsighted.”  See also: glasses prescription

low vision:

Low vision refers to when a person has poor vision in both eyes that cannot be fully corrected with glasses, contacts, or surgery.  The poor vision interferes with their daily life.  See also: visual impairment



A type of spherical error in which light is focused in front of the retina. This causes images far away to be blurry while those closer in may be clearer. In a glasses prescription, the spherical error will have a minus sign (-) in front of the number.  See also: glasses prescription



Another word for having myopia, also called “shortsighted.”  See also: glasses prescription


The part of the glasses that rests on the nose. Also called the bridge.


A vision condition in which the eyes make repeated involuntary movements from side to side which often results in reduced vision.

Learn more about nystagmus (from the American Optometric Association)

Read posts about nystagmus


occlusion therapy:

Treatment typically for amblyopia that involves covering the non-amblyopic (better) eye in order to stimulate and reinforce connections between the brain and the weaker eye. Occlusion therapy is typically done through patching, but can also be done through the use of an occluding contact, which blocks the vision in the eye in which it is worn.

ocular albinism:

A vision condition in which the iris and retina lack pigment. This leads to reduced vision, nystagmus, and light sensitivity. There are two broad types:

  • Ocular albinism (OA): in which only the iris and retina lack pigments
  • Oculocutaneous albinism (OCA): in which the iris, retina, skin, and hair lack pigments

Learn more about ocular albinism (from the Vision of Children Foundation)

Read posts about albinism and ocular albinism


An eye care provider, an ophthalmologist is a medical doctor who specializes in eye and vision care. Ophthalmologists complete medical school, they can prescribe glasses and contacts, and are licensed to practice medicine and surgery. Ophthalmologists will have “M.D.” credentials.


A technician trained to fit glasses frames, lenses, and contact lenses. They use prescriptions written by ophthalmologists or optometrists to order glasses or contacts. They cannot write prescriptions for vision correction.

optic nerve atrophy (ONA):

A vision condition due to damage to the optic nerve. Optic nerve atrophy can affect central and peripheral vision as well as color vision and can sometimes cause nystagmus.

Learn more about optic nerve atrophy (from the American Association of Pediatric Ophthalmology and Strabismus)

optic nerve hypoplasia (ONH):

A vision condition due to the underdevelopment of the optic nerve. Optic nerve hypoplasia is associated with visual impairment, nystagmus, and strabismus, though cases can be mild to severe.

Learn more about optic nerve hypoplasia (from the American Association of Pediatric Ophthalmology and Strabismus)


An eye care provider, an optometrist is a healthcare professional who provides primary vision care. The perform eye exams and can prescribe glasses and contacts and some medications for eye diseases. Optometrists receive a Doctor of Optometry (OD) degree after completing 4 years of optometry school. Some optometrists complete an optional residency in a particular area of specialty.


A health professional who is trained to help diagnose and treat eye movement problems. Orthoptists typically work with pediatric or neuro ophthalmologists.



A treatment in which an eye is covered by an eye patch. Patching is used as a treatment for amblyopia when they eye that sees better is covered to encourage the brain to use the amblyopic eye. Patching can also be used to protect an eye after surgery or injury.


Persistent Fetal Vasculature. An eye disorder in which the structures of the eye that were present during fetal development before birth do not go away they way they are supposed to. The resulting structural abnormality can lead to a variety of vision issues including strabismus, nystagmus, and amblyopia.

Read posts about PFV

Learn more about PFV (from American Society of Retina Specialists)


Being highly sensitive to bright lights.


Persistent hyperplastic primary vitreous, an alternate name for Persistent Fetal Vascular syndrome: see PFV


A material used for glasses lenses.  It is very impact-resistant and because of that, often recommended for children’s glasses. Polycarbonate is also relatively light weight. It provides full UVA/UVB protection.


An age-related vision issue that makes it difficult for people to focus clearly up close.  This is caused by a loss of elasticity in the lens that makes it difficult to accommodate and focus close up.  Presbyopia is often confused with hyperopia.  While both result in difficulties seeing up close, hyperopia can happen at any age and is due to the shape of the eye, while presbyopia typically develops in mid-40s and is a result of the aging process.


A prism can be added to a glasses lens to shift an image slightly in one direction or the other. This is sometimes used to alleviate double vision.


A drooping or falling of the upper eyelid.  Sometimes referred to as “lazy eye” (though typically, lazy eye refers to amblyopia).



Refraction is what happens when light is bent when it moves from one substance to another. You can see then when you put a straw in a glass of water and the straw appears to bend. During eye exams, “refraction” refers to the eye test in which the eye doctor measures the refractive error of the eyes — that is, the amount of myopia, hyperopia, and/or astigmatism that will then be used to determine the glasses prescription.

refractive error

When the shape of the eye does not focus the light correctly on to the retina, resulting in blurry vision.  Generally, refractive errors are what glasses are prescribed to correct.  The common refractive errors are:

  • Myopia (nearsightedness or shortsightedness): the light is focused in front of the retina
  • Hyperopia (farsightedness or longsightedness): the light is focused behind the retina
  • Astigmatism: the eye is irregularly shaped so that the light is focused in multiple spots.


The layer at the back of the eye that senses light and converts it to signals sent through the optic nerve to the brain.  Light needs to be focused correctly on the retina for a person to see clearly.

retinal detachment

When the retina (the layer of the eye that senses light) detaches from the back of the eye.  This is a medical emergency and must be treated immediately or it can lead to vision loss.  Symptoms include a dramatic increase in “floaters” and/or flashes of light in the eye.

Learn more about retinal detachment (From the National Eye Institute)


An eye cancer that forms in the retina.  Retinoblastoma typically develops in young children.  Treatment can include chemotherapy, radiation therapy, or other therapies to kill the cancer cells.  In some cases, the affected eye or eyes must be removed surgically.

Read posts about retinoblastoma

Learn more about retinoblastoma (from the American Cancer Society)

retinopathy of prematurity (ROP)

A vision condition that occurs usually due to a baby being born before 31 weeks gestation.   Retinopathy of prematurity can range from mild to severe.  Mild cases will improve on their own with no treatment.  More severe cases can lead to impaired vision or blindness.

Read posts about retinopathy of prematurity


A method of measuring the refractive error without needing a person to be able to respond verbally, because of this, retinoscopy is used quite often with children (especially pre-verbal or non-verbal children) to determine their glasses prescription.  The eye doctor shines a light into the eye through a lens and observes how the reflection of the light moves.



Another word for having myopia, also called “nearsighted.”  See also: glasses prescription

sphere (SPH):

The part of a glasses prescription that describes the correction needed for myopia (nearsightedness or shortsightedness) or hyperopia (farsightedness or longsightedness).  The sphere is listed in diopters. A plus number means correction for hyperopia and a minus number means correction for myopia. The higher the absolute value of the number (ignoring the + or -), the stronger the prescription is.

See also: Prescription


When the eyes are not aligned or straight. There are three types of strabismus:

    • esotropia: eyes pointed inward (crossed)
    • exotropia: eyes pointed outward (wall-eyed)
    • hypertropia: when one eye is pointed upward


“Squint” can mean a few different things, so it is best to be careful when using the term. It can mean keeping the eyes mostly closed or it can mean eyes that are not aligned (as in strabismus).



The part of the glasses that goes from the front of the frame to your ear. Sometimes referred to as the arm.
drawing of glasses with parts labeled: lenses, bridge (nose piece), and temples (ear pieces)



Unilateral descibes a condition that affects only one eye. For example, unilateral cataract means a cataract is present only in one eye. (Compare to bilateral)


A condition in which the uvea (the middle layer of the eye) is red and swollen.  Uveitis can damage eye tissues and lead to permanent vision loss.


vision therapy

A treatment for binocular vision issues.  Vision therapy is most often conducted under the supervision of an optometrist who has undergone additional vision therapy training.

Read more about vision therapy

visual impairment

A reduction in visual acuity that cannot be fully corrected with glasses or contacts.
The World Health Organization (WHO) has issued the following classifications of visual impairment:
  • Mild Vision Loss or Near-Normal Vision – 20/30 to 20/60 (6/9 to 6/18)
  • Moderate Visual Impairment – 20/70 to 20/160 (6/21 to 6/48)
  • Severe Visual Impairment – 20/200 to 20/400 (6/60 to 6/120)
  • Profound Visual Impairment – 20/500 to 20/1000 (6/150 to 6/300)
  • Near-Total Visual Impairment – less than 20/1000 (less than 6/300)
  • Total Visual Impairment – no light perception at all