Council of Trent
“Since Christ our Redeemer declared that to be truly His own body which He offered under the form of bread, it has, therefore, always been a firm belief in the Church of God, and this holy council now declares it anew, that by the consecration of the bread and wine a change is brought about of the whole substance of the bread into the substance of the body of Christ our Lord, and of the whole substance of the wine into the substance of His blood. This change the holy Catholic Church properly and appropriately calls transubstantiation.”
Source: Council of Trent Session XIII