The term "thermal energy" is used loosely in various contexts in physics and engineering. It can refer to several different well-defined physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, defined as a type of energy transfer (as is thermodynamic work); and the characteristic energy of a degree of freedom, ${\displaystyle k_{\mathrm {B} }T}$, in a system that is described in terms of its microscopic particulate constituents (where ${\displaystyle T}$ denotes temperature and ${\displaystyle k_{\mathrm {B} }}$ denotes the Boltzmann constant).