British thermal unit

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The quantity of heat required to raise the temperature of one pound of water from 60° to 61°F at a constant pressure of one atmosphere.
• noun The quantity of heat equal to ¹/₁₈₀ of the heat required to raise the temperature of one pound of water from 32° to 212°F at a constant pressure of one atmosphere.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit at a particular temperature (there are several different precise definitions) and a pressure of one atmosphere.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a unit of heat equal to the amount of heat required to raise one pound of water one degree Fahrenheit at one atmosphere pressure; equivalent to 251.997 calories