The physics technical term massive particle refers to a massful particle which has real non-zero rest mass (such as baryonic matter), the counter-part to the term massless particle. According to special relativity, the velocity of a massive particle is always less than the speed of light. When highlighting relativistic speeds, the synonyms bradyon (from Greek: βραδύς, bradys, "slow"), tardyon or ittyon are sometimes used to contrast with luxon (which moves at light speed) and hypothetical tachyon (which moves faster than light).