Abbreviation for "British Thermal Unit," the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. One Btu is equivalent to 1,054 joules or 252 calories. To gain an impression of the size of a Btu, the combustion of a barrel of oil yields about 5.6 x 10⁶ joule. A multiple of the Btu, the quad, is commonly used in discussions of national and international energy issues. The term quad is an abbreviation for one quadrillion, or 10¹⁵, Btu.