The amount of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit.
In ratings of oil heaters, furnaces, and other heating devices, you will frequently encounter reference to BTU. A manufacturer of a particular furnace, for example, will publish test data showing that his furnace is capable of generating so many BTUs of heat. This allows a prospective buyer to compare the performance of the furnace with that of a competitive model.
Fuels are also given a BTU rating. Gasoline, for example, has a higher BTU value than alcohols.