The Collaborative International Dictionary of English v.0.48:

Cranium \Cra"ni*um\ (kr?"n?-?m), n.; pl. E. Craniums (-?mz),
   L. Crania (-?). [NL., fr. Gr. ????; akin to ka`ra head.]
   The skull of an animal; especially, that part of the skull,
   either cartilaginous or bony, which immediately incloses the
   brain; the brain case or brainpan. See Skull.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cranium
    n 1: the part of the skull that encloses the brain [syn:
         cranium, braincase, brainpan]