The Collaborative International Dictionary of English v.0.48:

monstrance \mon"strance\ (m[o^]n*strans), n. [LL. monstrantia,
   fr. L. monstrare to show: cf. OF. monstrance. See Monster.]
   (R. C. Ch.)
   A transparent pyx, in which the consecrated host is exposed
   to view.
   [1913 Webster]

WordNet (r) 3.0 (2006):

monstrance
    n 1: proof by a process of argument or a series of proposition
         proving an asserted conclusion [syn: demonstration,
         monstrance]
    2: (Roman Catholic Church) a vessel (usually of gold or silver)
       in which the consecrated Host is exposed for adoration [syn:
       monstrance, ostensorium]