adj. Located or occurring below the arachnoid mater, often specifically between the arachnoid membrane and the pia mater
the GNU version of the Collaborative International Dictionary of English
adj. Situated under the arachnoid membrane.
The Century Dictionary and Cyclopedia
Situated beneath the arachnoid—that is, between that membrane and the pia mater: as, the subarachnoid space.
Subdural.
Word Usage
"Sharon Stone, as you know, Larry, had a very difficult situation in September of 2001, where she had a splitting headache and something called a subarachnoid hemorrhage, and she almost died."