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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.
Medical Definition of CAROTENOID. : any of various usually yellow to red pigments (as carotenes) found widely in plants and animals and characterized chemically by a long aliphatic polyene chain composed of eight isoprene units.â carotenoid adjective.efinition of CAROTENOID. : any of various usually yellow to red pigments (as carotenes) found widely in plants and animals and characterized chemically by a long aliphatic polyene chain composed of eight isoprene units. â carotenoid adjective.