Web like membrane that lies between the outer (and much thicker) dura mater and the deeper pia mater, and which covers the brain.
Note: The arachnoid membrane is separated from the pia mater by the subarachnoid space, in which the cerebrospinal fluid flows and is absorbed by the arachnoid villi.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 726 (https://doi.org/10.1515/pac-2015-1202)