1. The doctrine of the Catholic Church that bread and wine is altered into the body and blood of Jesus: When consecrated in the Eucharist, the concept is that food made from dough and then baked changes into the substance of the body of Christ, just as the fermented juice of grapes turns into the blood of Christ.
2. The conversion of one matter into another: Alchemists In the Middle Ages tackled the transubstantiation of turning lead into gold, but it never worked out and has never worked to this day!