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]

The Collaborative International Dictionary of English v.0.48:

elk \elk\ ([e^]lk), n. [Icel. elgr; akin to Sw. elg, AS. eolh,
   OHG. elaho, MHG. elch, cf. L. alces; perh. akin to E. eland.]
   (Zo["o]l.)
   A large deer, of several species. The European elk Alces
   alces (formerly Alces machlis or Cervus alces) is
   closely allied to the American moose. The American elk, or
   wapiti (Cervus Canadensis) the largest member of the deer
   family, has large, spreading antlers and is closely related
   to the European stag. See Moose, and Wapiti.
   [1913 Webster +PJC]

   Irish elk (Paleon.), a large, extinct, Quaternary deer
      (Cervus giganteus) with widely spreading antlers. Its
      remains have been found beneath the peat of swamps in
      Ireland and England. See Illustration in Appendix; also
      Illustration of Antler.

   Cape elk (Zo["o]l.), the eland. elk