Meaning

noun proof by a process of argument or a series of proposition proving an asserted conclusion

noun (Roman Catholic Church) a vessel (usually of gold or silver) in which the consecrated Host is exposed for adoration

originFrom Middle English mustraunce, from Old French mostrance, monstrance, from post-classical Latin mōnstrantia, from classical Latin mōnstrāns, present participle of mōnstrāre (“to show”).

Etymology from Wiktionary, CC BY-SA 3.0