1 : moving, operating, or acting quickly <a fast train> <a fast thinker>
2 : taking a short time <a fast trip>
3 : indicating ahead of the correct time <The clock is fast.>
4 : firmly placed <The plant's roots were fast in the ground.>
5 : not likely to fade <fast colors>
6 : totally loyal <fast friends>
Headscratcher: Someone fast can move very quickly while something held fast is stuck in one place and can't move anywhere at all!
synonyms: fast, rapid, and swift mean moving, proceeding, or acting with great speed. fast is used of the thing that moves. <He rode a fast horse.> rapid is used of the speedy movement itself. <The horse moved at a rapid pace.> swift suggests ease of movement along with great speed. <A swift horse can easily jump the fence.>