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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 is inflammation of the arachnoid, one of the membranes known as meninges that surround and protect the nerves of the central nervous system, including the brain and spinal cord.rachnoiditis is inflammation of the arachnoid, one of the membranes known as meninges that surround and protect the nerves of the central nervous system, including the brain and spinal cord.