When we talk about the true presence, we mean, in the words of the Catechism of the Catholic Church, that "the whole Christ is truly, really and substantially contained" (1374).

The Catechism speaks of Christ's presence in his word and in the community but emphasizes his "substantial" presence in the Eucharistic species.

Greek philosophy (Aristotle) differentiated substance from the accidents or the form of things. Substance meant the essence of something and not its shape or size, the means by which we perceive it through our senses.

St. Thomas Aquinas took these ideas and applied them to the sacraments. So we say that the appearances of the bread and wine remain the same but the substance changes and Christ becomes present.

Transubstantiation is a philosophical term which tries to explain how the change occurs. It was not used by the early Church. They were more interested in reflecting on Christ's actual presence in the bread and wine rather than the manner in which the change occurs.

However, this term continues to be used by the Catholic Church. Different terms have been suggested to express this reality but have been rejected by the Catholic Church.

Other Christians do not use this term, although some believe in the real presence. Many Christian churches consider that Christ is symbolically present but not actually present.

Catholics would not believe that other Christian churches have Christ present in the bread and wine in the way we do as the Catholic Church does not accept the ordination of other Christian churches as valid, except the Orthodox. However, that's a whole other story.