A hemianopia is a loss or decrease in sight in one half of the visual field in one or both eyes.

In the latter case, it can be heteronymous if the visual field concerned of each eye is different, or homonymous in the opposite case.

The altered visual field can be right lateral, left lateral (lateral involvement), vertical up, vertical down (this is called altitudinal impairment).

When this loss only affects a quarter and not half of a visual field, it is called a quadranopsia.