Cerebrospinal fluid. A watery fluid that flows around the ventricles (spaces) and surfaces of the brain, and also the surface of the spinal cord, to protect and nourish them. Abbreviated to CSF. It is continuously produced by the choroid plexus, and absorbed into the venous system. If more is produced than absorbed, or the flow of the fluid gets blocked, it causes a build-up of pressure, known as hydrocephalus.