Medical Definition of fat
1: animal tissue consisting chiefly of cells distended with greasy or oily matter—see brown fat
2a: oily or greasy matter making up the bulk of adipose tissueb: any of numerous compounds of carbon, hydrogen, and oxygen that are glycerides of fatty acids, are the chief constituents of plant and animal fat, are a major class of energy-rich food, and are soluble in organic solvents (as ether) but not in waterc: a solid or semisolid fat as distinguished from an oil
3: the condition of fatness : obesity
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