The Collaborative International Dictionary of English v.0.48:

Guillemot \Guil"le*mot`\, n. [F.] (Zool.)
   One of several northern sea birds, allied to the auks. They
   have short legs, placed far back, and are expert divers and
   swimmers.
   [1913 Webster]

   Note: The common guillemots, or murres, belong to the genus
         Uria (as U. troile); the black or foolish guillemot
         (Cepphus grylle, formerly Uria grylle), is called
         also sea pigeon and eligny. See Murre.
         [1913 Webster]

WordNet (r) 3.0 (2006):

guillemot
    n 1: small black or brown speckled auks of northern seas