The term 'thermal energy' is used differently, and often loosely, in different contexts. It refers to several distinct physical concepts, such as the internal energy, or as the enthalpy, of a body of matter and radiation; or as heat, defined as a type of energy transfer (as is thermodynamic work); or as the characteristic energy of a degree of freedom,
k
B
T
{\displaystyle k_{\mathrm {B} }T}
, in a system that is described in terms of its microscopic particulate constituents, where
T
{\displaystyle T}
denotes temperature and
k
B
{\displaystyle k_{\mathrm {B} }}
denotes the Boltzmann constant.