British Thermal Unit (BTU) is a traditional Imperial unit of energy. It is the amount of energy needed to cool or heat one pound of water through one degree Fahrenheit. To raise one pound of water by one degree Fahrenheit, requires one BTU of heat, equivalent to 1.055 × 103 joules. Greater heating capacity of the system reflects a high BTU rating.
A 12,000 Btu air conditioner can cool between 450 and 550 square feet of floor space.