The Collaborative International Dictionary of English v.0.48:

Wapiti \Wap"i*ti\, n. [Probably the Iroquois name. Bartlett.]
   (Zool.)
   The American elk (Cervus Canadensis). It is closely related
   to the European red deer, which it somewhat exceeds in size.
   [1913 Webster]

   Note: By some writers it is thought to be a variety of the
         red deer, but it is considered a distinct species by
         others. It is noted for the large, branching antlers of
         the male.
         [1913 Webster]

WordNet (r) 3.0 (2006):

wapiti
    n 1: large North American deer with large much-branched antlers
         in the male [syn: wapiti, elk, American elk, Cervus
         elaphus canadensis]
    2: common deer of temperate Europe and Asia [syn: red deer,
       elk, American elk, wapiti, Cervus elaphus]