The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

extensor
    n 1: a skeletal muscle whose contraction extends or stretches a
         body part [syn: extensor muscle, extensor] [ant:
         flexor, flexor muscle]