BTU stands for British Thermal Unit, which is a measure of energy used in the heating and cooling industry. Synonyms for BTU include heat unit, calorie, joule, kilowatt-hour, and therm. Heat unit measures the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. A calorie is the amount of energy required to raise the temperature of one gram of water by one degree Celsius. A joule is equal to the energy required to lift a one-kilogram weight by one meter against the force of gravity. A kilowatt-hour is the amount of energy used by a 1,000-watt appliance for one hour. Lastly, a therm is equal to 100,000 BTUs.