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British Thermal Unit, in science and engineering, a unit measurement of heat or energy, usually abbreviated as Btu or BTU. One Btu was originally defined as the quantity of heat required to raise the temperature of 1 lb (0.45 kg) of water from 59.5° F (15.3° C) to 60.5° F (15.8° C) at constant pressure of 1 atmosphere; for very accurate scientific or engineering measurements, however, this value was not precise enough. The Btu has now been redefined in terms of the joule as equal to 1055 joules; in engineering, a Btu is equivalent to approximately 0.293 watt-hour.
