British Thermal Unit (Btu)
Modern equivalent
1,055.06 joules
The quantity of heat required to raise the temperature of one pound of water by one degree Fahrenheit.
Defined in the language of the imperial system it grew out of, the Btu became the standard measure of heating capacity across British and American industry. It survives in the ratings of boilers, radiators, gas fires and air conditioning units, where equipment is still specified in Btu per hour even as the rest of the trade has moved to kilowatts. One kilowatt is roughly 3,412 Btu per hour.