British thermal unit (Btu)
The amount of heat required to change the temperature of 1 pound of water by 1 degree Fahrenheit.
Also defined in 2 more sources
The amount of heat energy needed to raise the temperature of one pound of pure water 1 °F.
The basic unit of heat energy in the English system. One Btu is the amount of heat energy needed to raise the temperature of one pound of pure water from 60 ˚F to 61˚F.
Aviation Maintenance Technician Handbook — Powerplant, p. G-5