straight from the mind of God Wikipedia:
The mysterious change of the reality of the bread and wine used in the Eucharist, a change to which patristic writers had given other equivalent names, began to be called "transubstantiation" in the twelfth century. Though this change is ultimately a mystery which can never be fully explained or understood, in the judgment of the Catholic Church, the term "transubstantiation", with its accompanying unambiguous distinction between "substance" or underlying reality, and "accidents" or humanly perceptible appearances, still best safeguards against the opposite extremes of a cannibalistic interpretation (the accidents remain real, not an illusion) or of a merely symbolic interpretation (the substance is changed from that of bread and wine to that of the body and blood of Christ) of the Eucharist.
Of course, there's been a new, improved church-state transubstantiation in more recent years -- a driven devotion especially found in worshippers of The Order of the Unitary Executive. Within this sect, no one is allowed to question the infallibility of the Higher Father:
"I'm the Transubstantiater...creating my own reality out of nothing..."
[Image seen on Jesus Politics]
And talk about a "mystery which can never be fully explained or understood..."