The Collaborative International Dictionary of English v.0.48:

Brainpan \Brain"pan`\, n. [Brain + pan.]
   The bones which inclose the brain; the skull; the cranium.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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