For the benefit of all others who are lost on this discussion on
ransubstantiation
Here is a short definition I managed to dig out.
Theology of transubstantiation
In this context, substance is a philosophical, not a chemical term.
It indicates what something is in itself. A hat's shape is not the
hat itself, nor is its colour the hat, nor is its size, nor its
softness to the touch, nor anything else about it perceptible to
the senses. The hat itself (the "substance") has the shape, the
colour, the size, the softness and the other appearances, but is
distinct from them. The appearances, which are referred to by the
philosophical term accidents are perceptible to the senses, the
substance is not.
When at his Last Supper Jesus said: "This is my body", what he held
in his hands had all the appearances of bread. However, though the
accidents of bread remained as before, the Roman Catholic Church
believes that the underlying reality was changed in accordance with
what Jesus said, that the substance of the bread was converted to
that of his body. And it believes that the same change of the
substance of the bread and a similar change of the substance of the
wine occurs at every celebration of the Eucharist.
The bread is not changed into Jesus: it is changed into his body.
But where Jesus' body is, there now is Jesus. Accordingly, when
bread is converted into his body, Jesus as a whole, body and blood,
soul and divinity, is present by concomitance. In the same way, the
wine is not changed into Jesus, but into his blood; but where his
blood is, Jesus, whole and entire, is present, not just his
blood.
The Roman Catholic Church accordingly believes that through
transubstantiation Christ is really, truly and substantially
present under the remaining appearances of bread and wine, and that
the transformation remains as long as the appearances remain. For
this reason the consecrated elements are preserved, generally in a
church tabernacle, for giving holy communion to the sick and dying,
and also for the secondary, but still highly prized, purpose of
adoring Christ present in the Eucharist.
The Roman Catholic Church considers the doctrine of
transubstantiation the best defence against what it sees as the
mutually opposed errors of, on the one hand, a merely figurative
understanding of the Real Presence of Christ in the Eucharist (the
change of the substance is real), and, on the other hand, an
interpretation that would amount to cannibalistic eating of the
flesh and corporal drinking of the blood of Christ (the accidents
that remain are real, not an illusion).
http://en.wikipedia.org/wiki/Transubstantiation
There are some disagreement on this issue, which Ironside and
Singaporemacros are debating about 
