Entry 1 sense Webster, 1913 Leptomeningitis /(lĕp`tō̇*mĕn`ĭn*jī"tĭs)/ · Lep·to·men·in·gi·tis · IPA 01 n. Inflammation of the pia mater or of the arachnoid membrane. 1. Inflammation of the pia mater or of the arachnoid membrane.(Med.)