According to Roman Catholic dogma, transubstantiation is the change of the substance of the Eucharistic elements -- bread and wine -- into the body and blood of Christ (although they retain the physical "accidents" -- i.e. appearance, taste, texture, etc. -- of bread and wine). In more colloquial use, the term refers to any belief that the elements of the Eucharist become the body and blood of Christ, with or without reference to "accidents" or other technical details specific to transubstantiation, strictly speaking.
The Roman Catholic Church holds that the belief that the elements of the Eucharist become the body and blood of Christ was given to the Apostles directly by Christ. The Synoptic Gospels present the words of Christ concerning the bread and wine at the Last Supper: "This is my body...This is my blood." (see Matthew 26:26-28) The Gospel of John records that Jesus said: "Unless you eat the flesh of the Son of Man and drink his blood you have no life in you." (John 6:53) St. Paul implies an identity between the apparent bread and wine and the body and blood of Christ when he writes: "Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of profaning the body and blood of the Lord." (1 Corinthians 11:27)
This understanding of the Eucharist was already well established in the Early Church. Ignatius of Antioch would appear to have accepted the concept when, in AD 106, he criticized those who "abstain from the Eucharist and the public prayer, because they will not admit that the Eucharist is the self-same body of our Savior Jesus Christ, which [flesh] suffered for our sins, and which the Father in His goodness raised up again" (Epistle to the Smyrnaeans 6, 7). Similarly, Ambrose of Milan countered objections to the doctrine, writing "You may perhaps say: 'My bread is ordinary.' But that bread is bread before the words of the Sacraments; where the consecration has entered in, the bread becomes the flesh of Christ." (The Sacraments, 333/339-397 A.D. v.2,1339,1340)
Under the influence of Scholasticism in the early Middle Ages, this concept became more technical in its terminology, as the scholastics inquired philosophically how and in what way the bread and wine become the body and blood of Christ. It was during this period that the term 'transubstantiation' was coined. Eventually, at the Fourth Lateran Council in 1215, and again at the Council of Trent (1545-1563) the terminology of transubstantiation was officially defined as dogma.
Contrast the belief held by some Protestant churches that Holy Communion merely symbolically commemorates Jesus' Last Supper with the disciples; this belief is known as "symbolism," "commemoration," or "transignification."
Some churches (notably some Lutheran communions) profess the doctrine of Consubstantiation, which holds that both the body and blood of Christ and the bread and wine are present in substance in the consecrated Eucharist. This doctrine agrees with Transubstantiation, and disagrees with Commemoration, that the real presence of Christ is in the Eucharist.
Anglican Churches generally use the term "real presence" without necessarily being more precise. Some Anglicans hold views nearly indistinguishable from transubstantiation, while others hold views closer to consubstantiation or other Protestant views. In 1622, there was a conference between William Laud, Archbishop of Canterbury, and the Jesuit John Fisher against the doctrine of transubstantiation. In 1684, Archbishop John Tillotson went as far as to speak of the "real barbarousness of this Sacrament and Rite of our Religion." For him, it was a great impiety to believe that people who attend Holy Communion "verily eat and drink the natural flesh and blood of Christ. And what can any man do more unworthily towards a Friend? How can he possibly use him more barbarously, than to feast upon his living flesh and bloud?" (Discourse against Transubstantiation, London 1684, 35.) This wideness of view has its roots in the sometimes violent controversies on religion during and after the reign of Henry VIII. During the reign of Elizabeth I, a more inclusive (some would say fuzzy) approach was adopted. Elizabeth's own response when questioned on this during the reign of Mary I is often quoted:
Christ was the word that spake it.
He took the bread and break it;
And what his words did make it
That I believe and take it.
The Eastern Orthodox Church, like the Catholic Church, teaches that the bread and wine truly become the body and blood of Christ. (The four-syllable word "metabole"/"metavole" is sometimes translated as "Greek Orthodox for 'transubstantiation'", although it actually means "change" or "alteration". Greek for "transubstantiation"--as in "an alteration specifically of the fundamental substance or essence" in the Roman Catholic sense--would be "metousiosis".) However, Orthodox theologians have tended to refrain from philosophical reflections such as those of the Medieval Scholastics. Rather, they prefer to refer to the Eucharist as a "mystery", with the full understanding beyond human comprehension. Most Orthodox theologians would prefer to say too little about the details and remain firmly within Holy Tradition, than say too much and possibly deviate from the truth.
Some Roman Catholic theologians interpreted transubstantiation as a change of meaning rather than a change of substance, however in 1965 Pope Paul VI mandated the retention of the original dogma of the twelth century.
Transubstantiation has historically resulted in accusations of cannibalism, beginning with the early church and more recently originating from radical sects. Ritual cannibalism has been reported to be used in some societies as a way to pay respect to, or gain physical or mental strength from the subject. However, such an origin in pagan religions is unlikely, given the early evidence of the doctrine in Christian history.