1. A congenital condition in which an abnormal accumulation of fluid in the cerebral ventricles causes enlargement of the skull and compression of the brain, destroying much of the neural tissue: "Hydrocephalus is also known as 'water on the brain' and it may be of sudden onset (acute hydrocephalus) or be slowly progressive (chronic hydrocephalus."
2. An accumulation of serous fluid within the cranium: especially, in infancy, because of an obstruction of the movement of cerebrospinal fluid, often causing a significant enlargement of the head: "When a person has hydrocephalus, he or she has an abnormal prominence of the forehead, brain atrophy, with mental deterioration and convulsions."