Blindness is a severe impairment or absence of vision. Blindness may be due to congenital defects (e.g., night blindness), accident, disease, or old age. Worldwide, the commonest cause of blindness is trachoma. In developed countries, it is most often due to severe diabetes (see diabetic retinopathy), glaucoma, or cataract, or degenerative changes associated with aging. Malnutrition (especially vitamin A deficiency) may cause blindness in children. Infant blindness can result if the mother has rubella (German measles) during pregnancy.

Transient blindness may occur if one is exposed to a vertical acceleration of more than about 5g. Cortical blindness is a disease of the higher perceptive centers in the brain concerned with vision: the patient may even deny blindness despite sever disability. Blindness due to cataract may be removed by laser treatment, or removal of the eye lens and the use of glasses. Prevention or early recognition and treatment of predisposing conditions is essential to save sight, as established blindness is rarely recoverable.