Thermal energy

Thermal energy is the part of the total, internal energy of a thermodynamic system or sample of matter that results in the system's temperature. The internal energy, also often called the thermodynamic energy, includes other forms of energy in a thermodynamic system in addition to thermal energy, namely forms of potential energy, such as the chemical energy stored in its molecular structure and electronic configuration, intermolecular interactions, and the nuclear energy that binds the sub-atomic particles of matter.
Microscopically, the thermal energy is the kinetic energy of a system's constituent particles, which may be atoms, molecules, electrons, or particles in plasmas. It originates from the individually random, or disordered, motion of particles in a large ensemble. The thermal energy is equally partitioned between all available quadratic degrees of freedom of the particles. These degrees of freedom may include pure translational motion in fluids, normal modes of vibrations, such as intermolecular vibrations or crystal lattice vibrations, or rotational states. In general, the availability of any such degrees of freedom is a function of the energy in the system, and therefore depends on the temperature.
When two thermodynamic systems with different temperatures are brought into diathermic contact, they exchange energy in form of heat, which is a conversion of thermal energy from the system of higher temperature to the colder system. This heat may cause work to be performed on each system, for example, in form of volume or pressure changes. This work may be used in heat engines to convert thermal energy into mechanical energy. When two systems have reached a thermodynamic equilibrium, they have attained the same temperature and the net exchange of thermal energy ceases.
Thermal energy is distinct from heat. In the strict use in physics, heat is a characteristic only of a process, i.e. it is absorbed or produced as an energy exchange, but it is not a static property of matter. Matter does not contain heat, but thermal energy. Heat is thermal energy in the process of transfer or conversion across a boundary of one region of matter to another.