Source : Webster's Revised Unabridged Dictionary (1913)

Dorsum \Dor"sum\, n. [L.]
   1. The ridge of a hill.

   2. (Anat.) The back or dorsal region of an animal; the upper
      side of an appendage or part; as, the dorsum of the
      tongue.

Source : WordNet®

dorsum
     n 1: the posterior part of a human (or animal) body from the neck
          to the end of the spine; "his back was nicely tanned"
          [syn: {back}]
     2: the back of the body of a vertebrate or any analogous
        surface (as the upper or outer surface of an organ or
        appendage or part); "the dorsum of the foot"
     [also: {dorsa} (pl)]