V

vascular dementia (VaD) A form of neurocognitive disorder characterized by sporadic, and progressive, loss of intellectual functioning caused by repeated infarcts, or temporary obstructions of blood vessels, which prevent sufficient blood from reaching the brain.

very low birthweight (VLBW) A body weight at birth of less than 3 pounds, 5 ounces (1,500 grams).

visual cliff An experimental apparatus that gives the illusion of a sudden drop-off between one horizontal surface and another.

vitality A measure of health that refers to how healthy and energetic—physically, intellectually, and socially—an individual actually feels.

voucher Public subsidy for tuition payment at a nonpublic school. Vouchers vary a great deal from place to place, not only in amount and availability but also in restrictions as to who gets them and what schools accept them.