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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 an inflammation or irritation of the arachnoid membrane of the meninges. The meninges are three layers of tissue covering the spinal cord and brain. Together, the brain and spinal cord make up the central nervous system. The arachnoid membrane is the middle layer of this covering.