Cerebrospinal Fluid (CSF):  A clear colorless fluid containing water, electrolytes, sugars and other molecules produced by the choroid plexus of the ventricles of the brain. CSF flows out of the fourth ventricle into the subarachnoid space which covers the brain and spinal cord.  CSF appears to have multiple functions including 1) acting as a buffer to cushion the brain and spinal cord against mechanical insults and trauma, 2) providing nutrition (sugar and other nutrients) to the adjacent neural tissues, 3) for communication through neurotransmitters, hormones and cytokines that pass into the CSF, and 4) for waste disposal of the central nervous system metabolism products.