Source : Webster's Revised Unabridged Dictionary (1913)

Extensor \Ex*ten"sor\, n. [L., one who stretches. See {Extend}.]
   (Anat.)
   A muscle which serves to extend or straighten any part of the
   body, as an arm or a finger; -- opposed to flexor.

Source : WordNet®

extensor
     n : a skeletal muscle whose contraction extends or stretches a
         body part [syn: {extensor muscle}] [ant: {flexor muscle}]