Glossary of Terms/Index

Up to your eyeballs with jargon? ​

Can’t see for all the specialist terminology? 

Looking for something in particular? 

Come and take a peek at our glossary of terms. Here you can find definitions for common ophthalmologic terms 

Table of Contents


labelled diagram of human eye

Acting as the outermost lens, it provides 65-75% of the eye’s focusing – or refracting – power. It is due to this pivotal role that defects in the shape or size of the cornea are the most common cause of sight problems in children. The cornea is entirely transparent, and unlike most other areas of the body, it houses no blood vessels.
Even singular capillaries could interrupt the ability for light to pass through uninterrupted. 
The cornea consists of 5 layers of tissue. These are:  

  1. Epitheliumthe outermost area of the cornea, the epithelium makes up only 1/10 of the total tissue in the cornea. Its primary function is as a shield against foreign bodies for the rest of the corneal layers and the inner eye; as a secondary function, it allows for oxygen and nutrient diffusion through the tears, which are then redistributed throughout the rest of the cornea. 
  2. Bowman’s Layer – a thin, transparent layer of the eye that sits just beneath the epithelium. Its function is widely debated, with some studies claiming that it has ‘no critical function in corneal physiology’. 
  3. Stroma the stroma is the largest layer of the cornea, comprising about 90% of its total thickness. The stroma is transparent, contains no blood vessels, and is made up mainly of water (which accounts for 78% of its volume) and collagen (16%). Collagen is responsible for giving the cornea its shape and elasticity, and it has been theorised that the stroma is responsible for producing ‘growth factors/cytokines’ that ‘contribute to wound healing’ within the cornea. 
  4. Descemet’s Membrane lying underneath the stroma, Descemet’s membrane was named after French physician, Jean Descemet. Again, this thin layer is composed of collagen, and contributes towards general corneal clarity and homeostasis, as well as acting as a barrier against infection. 
  5. EndotheliumThe final layer of the cornea, to which the Descemet’s membrane is attached. Endothelial cells are responsible for draining excess fluid from the stroma, maintaining its crucial shape and size. Once destroyed, endothelial cells are non-regenerative – sustained loss of endothelial cells will result in a fluid build-up in the stroma (corneal edema), eventually causing cloudy vision and potentially blindness. The only way to treat a case of chronic corneal edema is through corneal transplantation; in milder cases, it can disappear over time.

The iris is the coloured ring of muscle that sits outside the pupil. It is attached to the ciliary body, and is responsible for controlling the amount of light allowed into the eye, by constricting or dilating. This, in turn, ‘opens’ or ‘closes’ the pupil.

When there are low-light levels, the iris will dilate to allow more light into the eye; conversely, if it is extremely bright, the eyes will constrict to try and restrict the amount of light entering the eye. 

This is why you often will appear to have giant pupils when you take a photo with the flash on – because it’s dark, your irises dilate your pupils to allow more light into your eyes. This allows you to see better, however, your irises do not have enough time to react to the brightness of the camera flash. 


The clear, curved disk that sits behind the iris/pupil, dividing the eye into ‘posterior’ and ‘anterior’ sections. The lens is responsible for the rest of the eye’s refracting power that doesn’t come from the cornea. It is primarily made up of proteins (which account for 60% of its total mass, higher than almost any tissue in the body!). By changing its shape and size, the lens is able to move the focal point. In a healthy eye, this means that objects will always appear in focus – as the focal point is directly on the retina. 

Again, as with the cornea, the lens doesn’t contain any blood vessels (to allow light to pass through uninhibited); it receives any nutrients required, and removes waste products, through the aqueous humor. 

It is suspended by zonules (known as the zonules of Zinn) – a band of fibres made of protein – which connect it to the ciliary body. 

The lens has 4 constituent layers – these are the lens capsule, the epithelium, the cortex and the nucleus. Read more here. 

Optic Nerve

The optic nerve is a bundle of nerve fibres that is housed in the back of the eye socket, running into the optical canal to the back of the brain, where it enters the visual cortex. It is here that the electrical impulses generated by the retina are “read” and converted into thoughts.


The pupil is an opening in the centre of the eye, housed in the middle of the iris. The size of it (and through that, the amount of light that is let into the eye) is controlled by the iris.

It is not too dissimilar to the aperture of a camera.


the area of the eye that we use to “translate” photons (light) into neural impulses – converting visual information into an electrical signal our brain can recognise and act on. It is a membrane that sits at the back of the eye, made up of several layers of intricately specialised cells. 

The light is refracted by the cornea and lens, through the pupil, onto the retina.


Refractive Error

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A blanket term, which refers to a number of conditions – including hyperopia, myopia, astigmatism and presbyopia. This essentially refers to any defect in the size or shape of the eye, which results in the light being incorrectly refracted. The WHO estimates uncorrected refractive errors to be the number one cause of visual impairment in the world.

When light isn’t refracted correctly, it won’t hit the retina directly. The focal point of the photons (the point where the refracted beams converge, which is directly on the retina in a healthy individual) changes.

If this happens, our eyes cannot convey the required information to the brain. This results in images appearing out of focus or blurry. In some cases, sufferers may experience double-vision, or a distortion around bright lights.

Luckily, refractive errors are usually easy to correct. Using lenses (such as those in eyeglasses or contact lenses) can compensate for the incorrect refraction in the patient. In some cases, use of lasers or incisions on the eye can help correct the defect. The National Eye Institute has more information here. 


also known as hypermetropia, farsightedness, longsightedness; (View posts here!)
Another common vision problem – the opposite to myopia – wherein near objects appear blurred, whilst those far away appear in focus. It is estimated to affect 25% of the adult population – but again, prevalence is directly affected by age, with a much lower 8.4% incidence rate in children.

As with myopia, genetic factors appear to be a large predictor of diagnosis. The mechanisms of hyperopia are the opposite of myopia – so, that is, in the myopic eye the light is refracted in front of the retina; whereas in the hyperopic eye, it is refracted behind the retina. This can be due to a number of physiological defects – such as the eyeball being too small, or the cornea being too flat, thereby resulting in a lesser angle of refraction.

As with myopia, treatment for hyperopia usually comes in the form of corrective lenses (applied either as eyeglasses or contact lenses). Again, laser eye surgery to change the eye’s shape, or implanting permanent contact lenses are more long-term treatments for this condition.


also known as shortsightedness, nearsightedness; (View posts here!)

One of the most prevalent vision impairments across the world. Myopia is the opposite of hyperopia; it is a condition in which you can see objects close to you clearly, but objects far away appear blurred. It can be caused by a number of factors, and although prevalence can vary depending on age, it affects an estimated 32% of the British adult population.

Genetics can play a large role in diagnosis & prognosis, as the primary cause of myopia is a defect in either the size of the eyeball, or the shape of the cornea. Studies have also demonstrated a link between the level of “near-work” (for example, reading or writing) and the likelihood of a myopia diagnosis.

In a normal eye, light is usually refracted off the cornea to hit the retina directly. However, in myopic patients, the light from far objects hits in front of the retina instead. This results in the ‘blurred’ appearance.

It is corrected usually with lenses, which simply compensate for the ‘incorrect’ refraction. However, laser-eye surgery (which uses high-energy beams of light to cut incisions into the eye, changing its shape) or permanent lens implantation are also possible treatments. See the NHS page on myopia here.


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A term to describe an irregular corneal shape, resulting in blurred vision at any distance from the eye. The uneven shape results in a differing refraction of light depending on where the photons hit the eye. This can cause blurred, distorted or fuzzy vision in sufferers. In an astigmatic individual, instead of one, there are multiple focal points (points where the refracted beams of light converge)

The medical term for a condition more commonly known as lazy eye. Amblyopia affects roughly 3% of children, but is highly treatable. In the amblyopic individual, the connection between the brain and one of the eyes doesn’t develop properly. This results, over time, in the brain favouring the other eye more and more – which means the ‘lazy’ or amblyopic eye becomes weaker and weaker due to underuse.
Amblyopia can be caused due to genetic predisposition, or as a result of a secondary eye condition. In these cases, the other condition can weaken one eye’s vision, which can then lead to the brain “turning off” signals from that eye.
Treatment for amblyopia is easiest/most effective when it’s caught early – it is advised that if it is caught before age 7, then it is usually completely treatable. The usual prescription for amblyopia is either an eyepatch (to be worn over the stronger eye, to encourage the brain to use the weaker one) – or special eye-drops, designed to blur the stronger eye. This, again, forces the brain to use the other eye – thereby correcting the imbalance.

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This is the medical term for eyes that are normal, or suffering no defects. It essentially means that no matter how far away the object you are looking at is, there will be no distortion to the quality of the image. Regardless of how near or far it is, the light is refracted directly onto the retina.

So in theory this means that you can see something millions of miles away (assuming it was big enough, of course – like the stars at night), in just as much focus as something right in front of you. It means your visual acuity is ‘20/20’ – so, an object appears 20 feet away when it is 20 feet from you. By comparison, a myopic or hyperopic patient might have 20/40 vision (half the regular visual acuity) – so, something 20 feet away would appear 40 feet away to them.


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Presbyopia refers to age-induced degeneration of near-sight. It’s caused by the aging of the lens – which, as it grows older, loses much of its strength and flexibility (not too dissimilar to people!). This means that it cannot adjust its shape accordingly, so it struggles to move the focal point when switching from something far away to something near. It is currently predicted that 2 billion people suffer from presbyopia

It can cause, amongst other symptoms, blurred near-vision, headaches, eye-strain and visual fatigue. These can all contribute to making “near” tasks, such as reading/writing, increasingly difficult – which is why many older individuals might struggle to read their phone-screens without “adjusting” the distance first.

Treatment for Presbyopia is typically based around treating symptoms – this is usually a prescription of corrective lenses. The exact prescription is subject to change depending on the severity of presbyopia that is being displayed.

As more people become affected, there are a range of more permanent treatment options being explored. These are typically surgical. One such example is a corneal inlay implantation. This involves making a small incision in the cornea, and implanting a disc made of a water-permeable material into the stroma. This disc helps to increase the refractive power of the weakened lens, increasing the depth of focus in the eye.

Read more here!