The Collaborative International Dictionary of English v.0.48:

Ostensorium \Os`ten*so"ri*um\, Ostensory \Os*ten"so*ry\, n.; pl.
   L. -soria, E. -sories. [NL. ostensorium: cf. F.
   ostensoir. See Ostensible.] (R. C. Ch.)
   Same as Monstrance.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ostensorium
    n 1: (Roman Catholic Church) a vessel (usually of gold or
         silver) in which the consecrated Host is exposed for
         adoration [syn: monstrance, ostensorium]