Noun, pl. leptomeninges
1. A term used to refer to the pia mater and arachnoid mater, two of the membranes that surround the brain and the spinal cord. (wikipedia.org)
2. The innermost layers of the meninges; the pia mater and the arachnoid. (wiktionary.org)
3. The two innermost layers of the meninges; cerebrospinal fluid circulates between these innermost layers. (wordnetweb.princeton.edu)
4. Literally, thin meninges.