The Collaborative International Dictionary of English v.0.48:

manatee \man`a*tee"\, n. [Sp. manat['i], from the native name in
   Haiti. Cf. Lamantin.] (Zool.)
   Any species of Trichechus, a genus of sirenians; -- called
   also sea cow. [Written also manaty, manati.]
   [1913 Webster]

   Note: One species (Trichechus Senegalensis) inhabits the
         west coast of Africa; another (Trichechus Americanus)
         inhabits the east coast of South America, and the
         West-Indies. The Florida manatee (Trichechus
         latirostris) is by some considered a distinct species,
         by others it is thought to be a variety of Trichechus
         Americanus. It sometimes becomes fifteen feet or more
         in length, and lives both in fresh and salt water. It
         was hunted for its oil and flesh, and every species is
         now an endangered species.
         [1913 Webster]

WordNet (r) 3.0 (2006):

manatee
    n 1: sirenian mammal of tropical coastal waters of America; the
         flat tail is rounded [syn: manatee, Trichechus manatus]