relativistic particle
英[ˌrelətiˈvistik ˈpɑ:tikl]美[ˌrɛlətɪˈvɪstɪk ˈpɑrtɪkəl]
A relativistic particle is a particle which moves with a relativistic speed; that is, a speed comparable to the speed of light. This is achieved by photons and by tachyons to the extent that effects described by special relativity are able to describe those of such particles themselves.