Massive particle

A bradyon, also known as a tardyon or ittyon,^[1] is a particle that travels slower than light.^[2] All known particles that have mass are bradyons. The term "bradyon", from Greek: βραδύς (bradys, “slow”), was coined to contrast with "tachyon", from Greek: ταχύς (tachys, “swift, quick, fast, rapid”), which refers to hypothetical particles that travel faster than light.

Assuming that tachyons do not exist, 'bradyon' is a synonym for 'massive particle' (i.e. a particle having a non-zero mass). Any bradyon has a real rest mass, whereas any luxon has zero (both real and imaginary) rest mass. Tachyons (if they exist) have imaginary rest masses.

The words "tardyon" and "bradyon" are neologisms and are not yet in wide use, even within the field of physics.

References

1. ^ Bilaniuk, O.-M.P.; Sudarshan, E.C.G. (1969). "Particles beyond the Light Barrier". Physics Today 22 (5): 43–51. doi:10.1063/1.3035574. 
2. ^ Folman, R.; Recami, E. (1995). "On the Phenomenology of Tachyon Radiation". arXiv:hep-th/9508166 [hep-th].