brownish-yellow adipose tissue in the upper back, or interscapular, region of many mammals, most conspicuously in hibernating species and human newborns, composed of numerous innervated fat cells that can produce heat during cold stress; thermogenic tissue.
Babies use brown fat to burn calories and keep warm.
British Dictionary definitions for brown fat
tissue composed of a type of fat cell that dissipates as heat most of the energy released when food is oxidized; brown adipose tissue. It is present in hibernating animals and human babies and is thought to be important in adult weight control