Leptomeninge
The inner two meninges, the arachnoid and the pia mater, between which circulate the cerebrospinal fluid.
Literally thin meninges.
The morphology of the brain in meningococcal meningitis is said to be covered by exudate on the surface of and within the leptomeninges. Because the arachnoid is connected to the pia by cob-web like strands, it is structurally continuous with the pia, hence the name pia-arachnoid or leptomeninges.