Thermal conduction is the transfer of internal energy by microscopic diffusion and collisions of particles or quasi-particles within a body or between contiguous bodies. The microscopically diffusing and colliding objects include molecules, atoms, electrons, and phonons. They transfer disorganized microscopic kinetic and potential energy, which are jointly known as internal energy. Conduction takes place in all phases of ponderable matter, such as solids, liquids, gases and plasmas, but it is distinctly recognizable only when the matter is undergoing neither chemical reaction nor differential local internal flows of distinct chemical constituents. In the presence of such chemically defined contributory sub-processes, only the flow of internal energy is recognizable, as distinct from thermal conduction. A net flow of energy solely because of a temperature gradient is recognized as a flow of heat.