Glaucoma is an increase in the pressure of the half-set jelly-like fluid inside the eyeball that damages the eye and affects the vision. The eye is filled with a thick clear fluid (aqueous humour) that is slowly secreted by special cells within the eye, while in another part of the eye the fluid is removed, allowing a slow but steady renewal. If there is a blockage to the drainage of the fluid from the eye while new fluid continues to be secreted, the pressure inside the eye increases, and damage occurs to the light-sensitive retina at the back of the eye.
Glaucoma affects about 2% of the population.
Other conditions may also cause glaucoma including eye tumours, infections, injury, and in rare cases drugs (eg. steroids) may be responsible.