Elementary particle. Smallest unit of electromagnetic radiation. The energy of a photon is proportional to its frequency. One photon is emitted at  each transition of an electron from a level of elevated  energy to a lower level. The energy difference between the base level of valence electrons and the lowest elevated level usually corresponds to photons in the frequency range of visible light (frequency 800 THz - 400 THz, corresponding to a wavelength of 400 nm - 800 nm.) The colour of low pressure plasma radiation which is typical for any individual process gas is due to this electron transition.

Typical colour of a plasma
Photons emitted by stimulated atoms in a low pressure plasma are responsible for the characteristic colour of a process gas.