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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 a rare chronic pain condition that can occur when something - back surgery, for example - is done to your spine. With that said, the cause of arachnoiditis is not limited to back surgery.