Subarachnoid:The term "subarachnoid" literally means "beneath the arachnoid," which is the middle of the three membranes enveloping the central nervous system. In practical terms, it usually describes the area between the arachnoid membrane and the pia mater, the innermost layer. The subarachnoid space is a potential space, generally filled with cerebrospinal fluid. A hemorrhage into this space is known as a subarachnoid hemorrhage. The arachnoid membrane gets its name from the fine, spider-web-like strands that stretch from its lower surface through the cerebrospinal fluid in the subarachnoid space to the pia mater. The word "arachnoid" is derived from the Greek "arachne," meaning spider or cobweb, and "eidos," meaning resemblance, indicating its web-like appearance.
