Hemianopsia, or "Half Vision' is usually caused by a stroke

Hemianopsia is the medical term meaning loss of half of the visual field.

hemianopsia

Bitemporal hemianopia most commonly occurs as a result of tumors located at the mid-optic chiasm. Pituitary tumors (Pituitary Adenomas) and Craniopharyngiomas frequently result in a bitemporal hemianopsia.

The inability to see objects in half of the visual field of one or both eyes, which may be manifested by reading difficulties, problems walking through crowded areas, frequent accidents.