[TA] The two delicate layers of the meninges, the arachnoid mater and pia mater (vs. the tough pachymeninx or dura mater), considered together; by this concept, the arachnoid and pia are two parts of a single layer, much like the parietal and visceral layers of a membrane or bursa; although separated by the subarachnoid space they are connected via the arachnoid trabeculae and become continuous where the nerves and filum terminale exit the subarachnoid space (the cerebrospinal fluid-filled space bounded by the leptomeninges).arachnoid mater, pia mater. SYN: leptomeninges [TA], arachnoid mater and pia mater*, meninx tenuis, pia-arachnoid, piarachnoid.