A pleonasm is “a redundant word or expression”, like “free gift” or “true fact”. Wikipedia’s article on Pleonasm has a lot more information on redundancy in language, if you’re interested.
According to the Online Etymology Dictionary, pleonasm comes from Greek pleonasein, meaning “to be more than enough,” which comes from pleon “more”, which is related to poly-, meaning “many” or “much”.
Anyway, next time someone offers me a “free gift”, I’ll have to point out their redundant pleonasm.