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.
2. An increased quantity of cerebrospinal fluid (CSF) inside the brain that can result in increased pressure.

It is often the result of a disturbance (obstruction) in the normal cerebrospinal fluid (CSF) circulation or the over-production of CSF.

The diagnostic signs and symptoms of hydrocephalus depend upon the age of the person:

1. In infants the most obvious sign of hydrocephalus is usually an abnormally large head. (That is one reason a baby's head should be measured at every well-baby visit). Symptoms of hydrocephalus in an infant may include vomiting, sleepiness, irritability, an inability to look upwards, and seizures.
2. In older children and adults there is no head enlargement from hydrocephalus, but symptoms may include headache, nausea, vomiting and, sometimes, blurred vision. There may be problems with balance, delayed development in walking or talking, and poor coordination.