The Collaborative International Dictionary of English v.0.48:

Transubstantiation \Tran`sub*stan`ti*a"tion\, n. [LL.
   transubstantiatio: cf. F. transsubstantiation.]
   1. A change into another substance.
      [1913 Webster]

   2. (R. C. Theol.) The doctrine held by Roman Catholics, that
      the bread and wine in the Mass is converted into the body
      and blood of Christ; -- distinguished from
      consubstantiation, and impanation.
      [1913 Webster]

WordNet (r) 3.0 (2006):

transubstantiation
    n 1: the Roman Catholic doctrine that the whole substance of the
         bread and the wine changes into the substance of the body
         and blood of Christ when consecrated in the Eucharist
    2: an act that changes the form or character or substance of
       something [syn: transmutation, transubstantiation]