Cerebrospinal fluid: A watery fluid that is continuously produced and absorbed and that flows in the ventricles within the brain and around the surface of the brain and spinal cord. Abbreviated CSF. CSF is produced by the choroid plexus, a series of infolded blood vessels that project into the cerebral ventricles, and it is absorbed into the venous system. If production exceeds absorption, CSF pressure rises, and the result is hydrocephalus. This can also occur if the CSF pathways are obstructed, causing the fluid to accumulate. The CSF obtained during a lumbar puncture is analyzed to detect disease.