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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 the clumping together and tethering of the descending nerves to the inner lining of the water jacket that contains the fluid that supports brain and the cord in the spinal canal.rachnoiditis is the clumping together and tethering of the descending nerves to the inner lining of the water jacket that contains the fluid that supports brain and the cord in the spinal canal.