The term "thermal energy" is often used ambiguously in physics and engineering.[1] It can denote several different physical concepts, including:
Internal energy: The total energy contained within a body of matter or radiation.
Heat: Energy in transfer between a system and its surroundings by mechanisms other than thermodynamic work and transfer of matter.
The characteristic energy kBT associated with a single microscopic degree of freedom, where T denotes temperature and kB denotes the Boltzmann constant.
Mark Zemansky (1970) has argued that the term “thermal energy” is best avoided due to its ambiguity. He suggests using more precise terms like “internal energy” and “heat” to avoid confusion.[1] The term is, however, used in some textbooks.[2]