A deficiency of more than 10% under one’s desirable weight.
Weighing less than is medically advisable.
Being 10 to 15 percent below the standard weight for one’s height.
Body weight for height that is 15% to 20% below healthy weight; a body mass index below 19. By this standard, which is the one promoted both by the World Health Organization and the National Heart, Lung, and Blood Institute, a person who stands 5’7” tall is underweight if he or she weighs less than 120 lb.