Transubstantiation is a kind of mystical, unexplainable change in form, substance, or appearance. Medieval alchemists attempted the transubstantiation of lead into gold (unfortunately for them, that's not physically possible).
In some Christian teachings, particularly those of Roman Catholicism, transubstantiation is what happens ordinary bread and wine are transformed into the body and blood of Jesus Christ. To believers, although the food and drink appear to be the same after being consecrated by a priest, their true substance has transformed. This word comes from the Latin roots trans, "across or beyond," and substania, "substance."