What is the definition of a British Thermal Unit (BTU)?

The British Thermal Unit (BTU) is defined as the quantity of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit at a constant pressure of 1 atmosphere. This definition is crucial in the fields of heating and cooling, as it provides a clear measurement for the energy content in fuels and the energy required for heating systems.

Other definitions, such as the energy required to raise 1 pound of water by 1 degree Celsius, do not correctly represent the BTU because they reference a different temperature scale. Additionally, while some options mention heating water to boil or the energy needed to heat a gallon of water, these do not reflect the precise standard measurement of a BTU, which is specifically tied to the change of temperature of 1 pound of water by 1 degree Fahrenheit. Therefore, focusing on correct units and the specific conditions under which the measurement is taken is essential in identifying the accurate definition of a BTU.