Definitions for "Luxon"
A particle which travels at the speed of light. Apparently, only light particles (photons) themselves, since neutrinos, the only other massless particles thought to exist, now appear to have mass, and therefore always travel at least a little slower than the speed of light. Used as one of three terms to distinguish between particles which always travel slower than light ( tardyons), particles which always travel at the speed of light (luxons), and particles which always travel faster than light ( tachyons).
A luxon is a particle that always travels at the speed of light. Currently, the only known luxons are the massless gauge bosons: the photon (carrier of electromagnetism) and the gluon (carrier of the strong force). However, gluons are never observed as free particles, since they are confined within hadrons.