Glossary of Terms

This glossary of terms is not comprehensive, but serves as a guide for patients to understand commonly used ophthalmic terms.

View a glossary of medical terms below


Amblyopia: Also referred to as ‘lazy eye’. A failure of development of the part of the brain which processes vision, which can arise if the eye in question is misaligned with the dominant eye, or is significantly out of focus or is prevented from seeing clearly (e.g. by cataract) during the first 7 years or so of life when the visual system is still developing. It is usually reversible during this time by treating its cause and patching the other eye, but becomes irreversible once the visual system is mature.

Antimetabolite: The term given to a drug which retards cell division and slows down healing. In glaucoma surgery, antimetabolites are often used to prevent the drainage site from failing because of scarring.

Anti-VEGF drugs: Substances which block the action of Vascular Endothelial Growth Factor. They are effective in the treatment of choroidal neovascularisation.

Aqueous: The clear fluid which occupies the anterior chamber of the eye.

Astigmatism: A difference in the focal point of the eye in one meridian from another (e.g. between the horizontal and vertical planes). It can usually be corrected with a cylindrical (toric) lens.


Biologic therapeutic agent: A group of genetically engineered antibodies which target specific molecules involved in the immune system or the system which regulates inflammation and block their action. Anti-VEGF drugs belong to this group.


Choroid: The pigmented layer of the eye between the retina and the sclera. It contains a rich network of blood vessels.

Choroidal neovascular membrane: A network of blood vessels which originates from the choroid layer which comes to lie just under the retina. This results in leakage of fluid or bleeding into the retina. It is usually a complication of age-related macular degeneration and is popularly referred to as ‘wet macular degeneration’.

Ciliary body: A pigmented part of the eye, lying just behind the iris. One of its functions is to produce aqueous.

Cornea: The ‘front window’ of the eye.

Cycloplegic: A substance which paralyses the ciliary muscle. It is usually necessary to use this to obtain an accurate measurement of refractive error in younger children.


Descemet’s membrane: A layer of the cornea which lies just forward of the endothelium.

Dioptre: A unit of power of a lens, the reciprocal of the focal length of the lens.


Electromyography: A technique of recording electrical impulses from the contraction of a muscle. It is used to confirm the correct position of the needle when botulinum toxin is used to correct strabismus.

Endophthalmitis: Infection which involves the internal structures of the eye. It usually poses a serious threat to the visual function of the eye.

Endothelium: The layer of the cornea closest to the anterior chamber. It is a single layer of cells which does not regenerate after birth. Its function is to pump fluid out of the cornea. If it fails, the cornea becomes hazy (decompensates).

Epithelium (of the cornea): The layer of the cornea closest to the tear film.

Excimer laser: A type of laser which emits in the ultraviolet range of the spectrum. In ophthalmology, it is used for removing tissue from the cornea, particularly in refractive surgery.


Fovea: The centre of the retina, the area of greatest visual acuity.

Foveal burn: A laser burn in the fovea, which can occur as an adverse event during retinal laser treatment.


Glaucoma: A condition, usually characterised by raised pressure in the eye which causes damage to the optic nerve resulting in defects in the field of vision. It is treated by reducing the pressure in the eye.


Hyperopia (or hypermetropia): Long-sightedness, requiring a convex lens to focus the vision.


Lens capsule: The eye’s lens consists of tightly packed layers of transparent protein fibres contained within an elastic capsule. During cataract surgery, the layers of lens fibres are removed, but the posterior part of the lens capsule and its supporting zonular fibres are retained to act as a support for the lens implant.

Limbus: The region where the cornea and the sclera meet.


Macular oedema: The accumulation of fluid within the central area of the retina. This may occur as a complication of intraocular surgery, as a response to uveitis or as a complication of other retinal conditions such as diabetic retinopathy.

Microkeratome: A device which cuts a flap of the cornea prior to removal some of the thickness of the underlying corneal stroma with the excimer laser during refractive surgery.

Mydriatic: An eye drop or ointment which dilates the pupil.

Myopia: Short-sightedness, requiring a concave lens to focus the vision.


Neovascular glaucoma: A form of glaucoma which arises because of the growth of new blood vessels on the iris which close off the trabecular meshwork. It can be a complication of a number of conditions eg diabetic retinopathy.

NICE: National Institute for Health and Care Excellence

NOD: The National Ophthalmology Database. This is a database of outcomes of a very large number of ophthalmic surgical procedures, laser procedures and other treatments which have been derived from electronic medical records. It does not hold any patient-identifiable information, but it allows ophthalmologists to compare the results of their surgery or other treatments with that of other ophthalmologists.


Orthoptist: A health care professional who specialises in the assessment of strabismus and eye movement disorders. Orthoptists undertake a three year degree course and are registered with the Health and Care Professions Council.


Pars plana: A flat, rather featureless area of the ciliary body which contains relatively few blood vessels. Instruments used in vitreoretinal surgery are inserted in this region.

Posterior capsule opacification: Thickening of the lens capsule following cataract surgery. This is a healing response, which in some people may be sufficient to reduce the vision and require laser treatment to make an opening in the capsule at some point after cataract surgery.

Posterior capsule rupture: A break in the posterior capsule of the lens, usually as a complication of cataract surgery. It may allow vitreous to move forward into the anterior chamber of the eye.

Prostaglandin analogue: One of the groups of eye drops used to treat glaucoma. They work by increasing the rate at which aqueous passes out of the eye.


Refractive error: The amount of myopia, hyperopia or astigmatism.

Retina: The layer of the eye that contains the photoreceptors which detect light.


Schlemm’s canal: A small channel near the root of the iris which extends round the circumference of the eye. The trabecular meshwork filters aqueous into it, and from it the aqueous drains into veins which leave the eye.

Sclera: The ‘white’ of the eye. It forms the wall of the eye, except in the region of the cornea.

Strabismus Squint: A misalignment of the two eyes.

Stroma: The central layer of the cornea between the epithelium and endothelium. It makes up the majority of the thickness of the cornea.


Topical: Using eye drops or ointment.

Toric: A type of lens required to correct astigmatism.

Trabecular meshwork: A network of drainage channels located at the periphery of the anterior chamber of the eye, through which aqueous drains from the eye into the circulation.

Trabeculectomy: A surgical procedure to allow aqueous to flow from the anterior chamber of the eye to a space under the conjunctiva in a controlled fashion, in order to reduce the pressure in the eye.


Uveitis: Inflammation involving the pigmented layers of the eye (the iris, ciliary body and choroid).


Vasculitis (plural: vasculitides): A group of conditions characterised by the inflammation of blood vessel walls.

Vascular endothelial growth factor (VEGF): A protein which induces the growth of new blood vessels and is usually produced in response to impaired oxygen flow to tissues. It may be produced in increased quantity in diabetic retinopathy and age-related macular degeneration.

Vitrectomy: A surgical procedure to remove vitreous from the eye.

Vitreo-retinal surgery: Surgical procedures involving the vitreous or the retina.

Vitreous: A transparent substance with the consistency of egg-white which occupies the space inside the eye behind the lens.


Zonular fibres: The natural lens is suspended within the eye by elastic fibres around its circumference. These, and the lens capsule, are retained during the cataract surgery to allow the lens implant to be supported in the position that the original lens occupied.