Guillemots is the common name for several species of seabird in the auk family (part of the order Charadriiformes). In British use, the term comprises two genera: Uria and Cepphus. In North America the Uria species are called "murres" and only the Cepphus species are called "guillemots". This word of French origin apparently derives from a form of the name William, cf.
The two living species of Uria, together with the Razorbill, Dovekie and the extinct Great Auk, make up the tribe Alcini. They have distinctly white bellies, thicker, and longer bills than Cepphus and form very dense colonies on cliffs during the reproductive season.
The three living species of Cepphus form a tribe of their own: Cepphini. They are smaller than the Uria species and have black bellies, rounder heads and bright red feet.