What is British Thermal Unit?

Legal Definition
The British thermal unit (Btu or BTU) is a traditional unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is part of the British Imperial system of units. Its counterpart in the metric (SI) system is the calorie, which is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius. Heat is now known to be equivalent to energy, for which the metric unit is the joule; one BTU is about 1055 joules. While units of heat are often supplanted by energy units in scientific work, they are still important in many fields. As examples, in the United States the price of natural gas is quoted in dollars per million BTUs. Chemical bond energies are often given in calories per mole of substance.
-- Wikipedia
Legal Definition
A traditional unit of energy equal to about 1,055 joules. It is approximately the amount of energy needed to heat 1 pound (0.454 kg) of water (i.e. exactly one tenth of a UK gallon, or around 0.1198 US gallons) from 39 to 40