British thermal unit

The British thermal unit (BTU or Btu) is a unit of energy used in the United States of America, particularly in the power, steam generation and heating and air conditioning industries. Although it is still used 'unofficially' in metric English-speaking countries (such as Canada, the United Kingdom, and sometimes in New Zealand), it is increasingly an outmoded and outdated unit of measure. Elsewhere (and always in scientific use) the BTU has been replaced by the SI unit of energy, the joule (J).
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Noun
1. a unit of heat equal to the amount of heat required to raise one pound of water one degree Fahrenheit at one atmosphere pressure; equivalent to 251.997 calories
(synonym) BTU, B.Th.U.
(hypernym) work unit, heat unit, energy unit
(part-holonym) therm

(See also Conversion Factor) The amount of energy required to raise the temperature of 1 pound of water 1 degree Fahrenheit (F) at or near 39.2 degrees F and 1 atmosphere of pressure. One British Thermal Unit (BTU) is about equal to the heat given off by a blue-tip match. (DOE3) (DOE4)

Unit of heat energy equal to the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at sea level.

The quantity of heat required to raise the temperature of one pound of water one degree of Fahrenheit at or near 39.2 degrees Fahrenheit.