- the amount of heat required to raise the temperature of 1 pound (0.4 kilogram) of water 1°F. Abbreviation: Btu, BTU, B.t.u., B.T.U., B.th.u.
- a unit of heat in the fps system equal to the quantity of heat required to raise the temperature of 1 pound of water by 1°F. 1 British thermal unit is equivalent to 1055.06 joules or 251.997 calories
