If left untreated, glaucoma can cause blindness. But if it is diagnosed and treated early enough, further damage to vision can be prevented. Glaucoma occurs when the drainage tubes within the eye become slightly blocked. This prevents eye fluid from draining properly. When the fluid cannot drain properly, pressure builds up. This is called intraocular pressure. This can damage the optic nerve, which connects the eye to the brain, and the nerve fibres from the retina (the light-sensitive nerve tissue that lines the back of the eye).
Definition from NHS Choices: Glaucoma