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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.
Sections. The arachnoid mater is one of three membranes that cover the brain and the spinal cord. The arachnoid is the middle layer â situated between the outermost layer, the dura mater, and the innermost layer, the pia mater. Tiny fibers extend off the arachnoid membrane and attach to the pia mater.