A visual disorder where a patient can only see to one side, either the right or left. It may occur due to stroke, tumor, or trauma, which causes a lesion to the brain's visual pathways from half of the visual fields of both eyes. The left side of the brain has the paths for the right visual fields of both eyes, while the right side of the brain has the pathways for the left hemifields of both eyes. Damage to either of the visual pathways leads to the loss of the corresponding visual field.