Chapter 18 — Part II: The Sacraments — The Holy Eucharist
Section 107. This admirable change, as the Council of Trent teaches, the Holy Catholic Church most appropriately expresses by the word transubstantiation. Since natural changes are rightly called transformations, because they involve a change of form; so likewise our predecessors in the faith wisely and appropriately introduced the term transubstantiation, in order to signify that in the Sacrament of the Eucharist the whole substance of one thing passes into the whole substance of another.