late 14c., "change of one substance to another," from Medieval Latin trans(s)ubstantiationem (nominative trans(s)ubstantio), noun of action from trans(s)ubstantiare "to change from one substance into another," from Latin trans- "across" (see trans-) + substantiare "to substantiate," from substania "substance" (see substance). Ecclesiastical sense in reference to the Eucharist first recorded 1530s.
According to the traditional teaching of the Roman Catholic Church, the presence of Jesus in the sacrament of Communion. Through transubstantiation, the bread and wine consumed by worshipers become the body and blood of Jesus when a priest, acting on Jesus' behalf, speaks the words “This is my body” and “This is my blood” over them.
Note: Transubstantiation was the focus of a great controversy during the Reformation, because most other groups of Christians do not maintain this doctrine. They usually hold that the body and blood of Jesus are only symbolically present in the bread and wine or that the bread and wine are the body and blood of Jesus and bread and wine at the same time.