The acronym for British Thermal Unit is Btu. It is a traditional unit of heat energy, originally defined as the amount of heat needed to raise one pound of water by one degree Fahrenheit.
One Btu is approximately 1,055 joules, although slightly different historical definitions produce values that vary by a small amount. The unit belongs to the United States customary and imperial measurement traditions and remains common in heating, cooling, building insulation, and natural-gas pricing.
In everyday specifications, air conditioners and furnaces are often described in Btu per hour, written Btu/h. That expression measures power—the rate of heat transfer—not merely an amount of energy. A 12,000 Btu/h air conditioner, for example, transfers heat at roughly the rate traditionally called one ton of refrigeration.
The capitalization “Btu” is the standard technical form, though “BTU” is widespread in commercial writing. Btu should also not be confused with the obsolete British “Board of Trade unit,” which referred to a kilowatt-hour.