(ŭn′dĕr-wāt″)
[Pronunciation]

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.
In children, underweight can be defined as having a weight-for-height that is less than the 5^th percentile for one's age.