The Collaborative International Dictionary of English v.0.48:

Hydrocephalus \Hy`dro*ceph"a*lus\, n. [NL., fr. Gr. ?
   hydrocephalus; "y`dwr water + ? head.] (Med.)
   An accumulation of liquid within the cavity of the cranium,
   especially within the ventricles of the brain; dropsy of the
   brain. It is due usually to tubercular meningitis. When it
   occurs in infancy, it often enlarges the head enormously.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hydrocephalus
    n 1: an abnormal condition in which cerebrospinal fluid collects
         in the ventricles of the brain; in infants it can cause
         abnormally rapid growth of the head and bulging fontanelles
         and a small face; in adults the symptoms are primarily
         neurological [syn: hydrocephalus, hydrocephaly]