Source : Webster's Revised Unabridged Dictionary (1913)

Flexor \Flex"or\, n. [NL.] (Anat.)
   A muscle which bends or flexes any part; as, the flexors of
   the arm or the hand; -- opposed to {extensor}.

Source : WordNet®

flexor
     n : a skeletal muscle whose contraction bends a joint [syn: {flexor
         muscle}] [ant: {extensor muscle}]