subarachnoid space

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The space between the arachnoid membrane and pia mater that is filled with cerebrospinal fluid and contains the large blood vessels that supply the brain and spinal cord.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy The space between the arachnoid mater and the pia mater that surround the brain and spinal cord.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a space in the meninges beneath the arachnoid membrane and above the pia mater that contains the cerebrospinal fluid