(n)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
The Collaborative International Dictionary of English (GCIDE) v.0.53Collaborative International Dictionary (GCIDE)
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 ]