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British thermal unit-a unit of energy associated with the creation of heat. Prior to 1929, it was defined as the amount of heat required to raised the temperature of 1 pound of water 1 degree Fahrenheit. In 1929, it was redefined as being equivalent to 251.996 calories, 778.26 ft-lb, or in electrical units, approximately one-third watt-hours. In the United States, the BTU is a standard measure of the heat-producing capabilities of various heating appliances, furnaces, and fuels.