cerebrospinal fluid

Short definition from Wiktionary

Noun

1. 1
   A clear bodily fluid in the form of a very pure saline solution with microglia that occupies the subarachnoid space in the brain (between the skull and the cerebral cortex), and which acts as a cushion or buffer for the cortex.
2. 2
   A bodily fluid filling the third ventricle and surrounding the interthalamic adhesion.