Hydrocephalus is a congenital or acquired condition characterised by marked dilatation of the cerebral ventricles, accompanied by the accumulation of cerebrospinal fluid within the skull. More commonly it results from impaired absorption of CSF than from excessive secretion.

Hydrocephalus may be a result of an obstruction outside the ventricular system or an intra-ventricular blockage; termed communicating and non-communicating hydrocephalus respectively.

Another condition, hydrocephalus ex vacuo, manifests as enlarged ventricles because of destruction of cerebral tissue and the consequent expansion of the ventricles.