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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.
Arachnoiditis: Introduction. Arachnoiditis: Arachnoiditis is an inflammatory response of the arachnoid, one of three coverings, or meninges, that envelop the brain and spinal ... more about Arachnoiditis. Arachnoiditis: A progressive disorder where the arachnoid membrane becomes inflamed and the brain and spinal cord may also become inflamed.