Scientifically, thermal mass is equivalent to thermal capacity or heat capacity, the ability of a body to store thermal energy. It is typically referred to by the symbol C th, and its SI unit is J/K or J/°C (which are equivalent). Thermal mass may also be used for bodies of water, machines or machine parts, living things, or any other structure or body in engineering or biology. In those contexts, the term "heat capacity" is typically used instead.