Regarding the place beneath the arachnoid, the middle of three membranes that cover the central nervous system.

In practice, the subarachnoid space is between the arachnoid and the pia mater, the innermost membrane surrounding the central nervous system.

The subarachnoid area normally contains cerebrospinal fluid. A subarachnoid hemorrhage is a bleeding into this space.

The arachnoid is named for its delicate, spider-web-like filaments that extend from its undersurface through the cerebrospinal fluid in the subarachnoid space to the pia mater (delicate and highly vascular membrane immediately covering, or enveloping, the brain and spinal cord).