The Collaborative International Dictionary of English v.0.48:

Megalocephalia \Meg`a*lo*ce*pha"li*a\, Megalocephaly
\Meg`a*lo*ceph"a*ly\, n. [NL. megalocephalia, fr. Gr. ? having a
   large head.] (Med.)
   The condition of having an abnormally large head; called also
   megacephaly. -- Meg`a*lo*ce*phal"ic, a.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

megacephaly
    n 1: an abnormally large head; differs from hydrocephalus
         because there is no increased intracranial pressure and the
         overgrowth is symmetrical [syn: macrocephaly,
         megacephaly, megalocephaly]