The Collaborative International Dictionary of English v.0.48:

Cepheus \Ce"pheus\, n. (Astron.)
   A northern constellation near the pole. Its head, which is in
   the Milky Way, is marked by a triangle formed by three stars
   of the fourth magnitude. See Cassiopeia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Cepheus
    n 1: (Greek mythology) king of Ethiopia and husband of
         Cassiopeia
    2: a faint constellation in the northern hemisphere near
       Cassiopeia and the pole star