Wiktionary
arachnoid mater
n. (context anatomy English) The middle layer of the meninges. A membrane that protects the brain and spinal cord below the dura mater and above the subarachnoid space and the pia mater.
Wikipedia
Arachnoid mater
The arachnoid mater is one of the three meninges, the protective membranes that cover the brain and spinal cord. The arachnoid mater is a derivative of the Neural crest mesectoderm in the embryo.