narwhal(n = noun.animal) monodon monoceros, narwal, narwhale - small Arctic whale the male having a long spiral ivory tusk;
is a kind of whale
is a member of genus monodon, monodon

   An arctic cetacean (Monodon monocerous), about twenty feet long. The male usually has one long, twisted, pointed canine tooth, or tusk, projecting forward from the upper jaw like a horn, whence it is called also sea unicorn, unicorn fish, and unicorn whale. Sometimes two horns are developed, side by side. [1913 Webster]

narwhal, n. an Arctic white whale, Monodon monoceros, the male of which has a long straight spirally fluted tusk developed from one of its teeth. Also called BELUGA.

Etymology:
Du. narwal f. Da. narhval f. hval whale: cf. ON n{aacute}hvalr (perh. f. n{aacute}r corpse, with ref. to its skin-colour)