The term massive particle refers to particles which have real non-zero rest mass. According to special relativity, their velocity is always lower than the speed of light. The synonyms bradyon (from , 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).

The term massive particle refers to particles which have real non-zero rest mass. According to special relativity, their velocity is always lower than the speed of light. The synonyms bradyon (from , 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).