The Collaborative International Dictionary of English v.0.48:

Caribou \Car"i*bou\ (k[a^]r"[i^]*b[=oo]), n. [Canadian French.]
   (Zool.)
   The American reindeer, especially the common or woodland
   species (Rangifer Caribou).
   [1913 Webster]

   Barren Ground caribou. See under Barren.

   Woodland caribou, the common reindeer (Rangifer Caribou)
      of the northern forests of America.
      [1913 Webster]

WordNet (r) 3.0 (2006):

caribou
    n 1: Arctic deer with large antlers in both sexes; called
         `reindeer' in Eurasia and `caribou' in North America [syn:
         caribou, reindeer, Greenland caribou, Rangifer
         tarandus]