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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.
What Is an Arachnoid Cyst? An arachnoid cyst usually develops in the head, but may also develop around the spinal cord. It is named an arachnoid cyst because it occurs in the space between the brain (or spinal column) and the arachnoid membrane. This is one of three membrane layers surrounding the brain and spine. If in the head, the cyst will grow between the brain and skull or in pockets around the brain called ventricles.