The Collaborative International Dictionary of English v.0.48:

Grebe \Grebe\ (gr[=e]b), n. [F. gr[`e]be, fr. Armor. krib comb;
   akin to kriben crest, W. crib comb, crest. So called in
   allusion to the crest of one species.] (Zool.)
   One of several swimming birds or divers, of the genus
   Colymbus (formerly Podiceps), and allied genera, found in
   the northern parts of America, Europe, and Asia. They have
   strong, sharp bills, and lobate toes.
   [1913 Webster]

WordNet (r) 3.0 (2006):

grebe
    n 1: small compact-bodied almost completely aquatic bird that
         builds floating nests; similar to loons but smaller and
         with lobate rather than webbed feet