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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.
Medical Definition of SUBARACHNOID SPACE. : the space between the arachnoid and the pia mater through which the cerebrospinal fluid circulates and across which extend delicate trabeculae of connective tissue. the space between the arachnoid and the pia mater through which the cerebrospinal fluid circulates and across which extend delicate trabeculae of connective tissue.