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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.
An arachnoid cyst is a fluid-filled cyst that that occurs in the arachnoid membrane around the brain. While completely benign, and not a tumor per se, they can cause symptoms by compression of adjacent nervous system structures.