The Collaborative International Dictionary of English v.0.48:

Depressor \De*press"or\, n.
   1. One who, or that which, presses down; an oppressor.
      [1913 Webster]

   2. (Anat.) A muscle that depresses or tends to draw down a
      part.
      [1913 Webster]

   Depressor nerve (Physiol.), a nerve which lowers the
      activity of an organ; as, the depressor nerve of the
      heart.
      [1913 Webster]

WordNet (r) 3.0 (2006):

depressor
    n 1: any skeletal muscle that draws a body part down [syn:
         depressor, depressor muscle]
    2: any nerve whose activity tends to reduce the activity or tone
       of the body part it serves [syn: depressor, depressor
       nerve]
    3: a device used by physician to press a part down or aside