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Btu:see British thermal unitBritish thermal unit,
abbr. Btu, unit for measuring heat quantity in the customary system of English units of measurement, equal to the amount of heat required to raise the temperature of one pound of water at its maximum density [which occurs at a temperature of 39.
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British thermal unit
The amount of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit. Abbr. Btu.