British thermal unit.
Related Terms 
ENTHALPY
A thermal property of a fluid which is a function of state and is defined as the sum of stored mechanical potential energy and internal energy. It is generally expressed in Btu per pound of fluid (joules per kilogram).

HEAT RELEASE
The total quantity of thermal energy above a fixed datum introduced into a furnace by the fuel, considered to be the product of the hourly fuel rate and its high heating value, expressed in Btu per hour per cubic foot of furnace volume or square foot of heating surface.

INPUT RATING
The fuel burning capacity of a burner at sea level in Btu per hour as specified by the manufacturer.

SPECIFIC HEAT
The quantity of heat, expressed in Btu (joule) required to raise the temperature of 1 lb. (kilogram) of a substance 1oF (oC)

BTU (BRITISH THERMAL UNIT)
A standard measure of energy in the British unit system. 1 Btu is the amount of heat required to raise the temperature of a liquid by 1 degree.

CALORIMETERA device used to measure heat quantities, specifically, the BTU value of fuel oil. 