Medical definition of Glaucoma : It is a type of optic neuropathy, frequently associated with raised intra-ocular pressure and visual field defects.
In Simple terms : Gradual increase of pressure in the eyeball causes damage to the nerves carrying the signal from the eye. As a result of this, "field-of-vision" of the impacted person starts becoming narrower. In the early stages, Glaucoma patient might have a good straight vision, but his/her peripheral vision would be impacted to a large extent. If this condition is not treated early and Glaucoma progresses, patient may lose his/her vision completely.