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Definition of arachnoid. 1  1 : of or relating to a thin membrane of the brain and spinal cord that lies between the dura mater and the pia mater. 2  2 : covered with or composed of soft loose hairs or fibers.
Arachnoid cysts are fluid-filled sacs that occur on the arachnoid membrane that covers the brain (intracranial) and the spinal cord (spinal). There are three membranes covering these parts of the central nervous system: the dura mater, arachnoid, and pia mater. Arachnoid cysts appear on the arachnoid membrane, and they may also expand into the space between the pia mater and arachnoid membranes (subarachnoid space).