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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 disease characterized by an acute inflammatory stage that occurs in the dura (exterior) and the arachnoid (interior), two of the three membranes that cover and protect the brain, the spinal cord and the nerve roots.