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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.
The arachnoid mater has delicate, cobweb-like filaments that are attached to the top of the pia mater, connecting them structurally. Combined, the two layers are known as the leptomeninges. The void in between the arachnoid and pia mater is called the subarachnoid space. This space holds the central nervous systemâs cerebrospinal fluid.