46 To avoid any misunderstanding of this type of presence, which goes beyond the laws of nature and constitutes the greatest miracle of its kind, (50) we have to listen with docility to the voice of the teaching and praying Church. Her voice, which constantly echoes the voice of Christ, assures us that the way in which Christ becomes present in this Sacrament is through the conversion of the whole substance of the bread into His body and of the whole substance of the wine into His blood, a unique and truly wonderful conversion that the Catholic Church fittingly and properly calls transubstantiation. (51) As a result of transubstantiation, the species of bread and wine undoubtedly take on a new signification and a new finality, for they are no longer ordinary bread and wine but instead a sign of something sacred and a sign of spiritual food; but they take on this new signification, this new finality, precisely because they contain a new "reality" which we can rightly call ontological. For what now lies beneath the aforementioned species is not what was there before, but something completely different; and not just in the estimation of Church belief but in reality, since once the substance or nature of the bread and wine has been changed into the body and blood of Christ, nothing remains of the bread and the wine except for the species—beneath which Christ is present whole and entire in His physical "reality," corporeally present, although not in the manner in which bodies are in a place.
Source: Mysterium Fidei (Vatican.va)