Thermal emission (or thermal radiation) is electromagnetic radiation (EMR) emitted by something because of its heat, such as from the activity at the molecular level creating photons. The well-known type of thermal emission, black-body radiation results when the source is in thermodynamic equilibrium, and often the terms thermal emission and thermal radiation presuming that case. Thermodynamic equilibrium and implies the source is at a defined, constant temperature, and if a gas, implies the particle velocity is described by the Maxwell-Boltzmann distribution, The Planck function describes the resulting spectrum (black-body spectrum), and the resulting EMR is termed Planckian.
Thermal emission from a source not in thermodynamic equilibrium diverges from the black-body spectrum (thus is non-Planckian, NP), but if it is close to equilibrium, the divergence may be small, and the black-body spectrum is often taken to be a good approximation. Kirchhoff's laws characterize the results of some situations. Thermal emission from an optically thin plasma can result in thermal bremsstrahlung, another case where the spectrum is non-Planckian.