British Thermal Unit (thermochemical Definition)

The $\textit{British Thermal Unit (thermochemical definition)}$ ($BTU_{th}$), is a traditional unit of energy equal to about $1.0543502645 kilojoule$. It is approximately the amount of energy needed to heat 1 pound (0.454 kg) of water from $39^{\circ}\text{F}$ ($3.9^{\circ}\text{C}$) to $40^{\circ}\text{F}$ ($4.4^{\circ}\text{C}$). The unit is most often used in the power, steam generation, heating and air conditioning industries. In scientific contexts the BTU has largely been replaced by the SI unit of energy, the $joule$.