Arachnoid is a noun referring to a delicate, spiderweb-like membrane (the arachnoid mater) that lies between the dura mater and the pia mater surrounding the brain and spinal cord. In anatomy discussions it denotes this specific meningeal layer, often in phrases like arachnoid mater and arachnoid space. The term is used in medical and academic contexts and is pronounced with emphasis on the third syllable.
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