Simple Definition -
Transubstantiation is the act by which Christians believe the bread and wine is converted into the substance of the Body and Blood of Jesus Christ. Through transubstantiation the Resurrected Jesus is present in the Eucharist.
The word 'Transubstantiation' is created from two words, the first being trans, meaning "across" and "through", and the second being substantia, referring to a "substance". The term "transubstantiate" means changing from one substance to another.
This change is brought about in the Eucharistic prayer through the word of Christ and the action of the Holy Spirit.
In paragraph 1376-1377 of the Catechism of the Catholic Church:
'Transubstantiation means the change of the whole substance of bread into the substance of the Body of Christ and of the whole substance of wine into the substance of his Blood.'
Transubstantiation is a change in a substance from one thing to another. For example, the bread into the Body of Jesus.
'The outward characteristics of bread and wine, that is the “Eucharistic species”, remain unaltered.'
This meaning that the appearance of the Eucharist remains the same although the substance is transformed into the Body and Blood of Jesus.
Roman Catholics believe that through transubstantiation, the risen and resurrected Christ becomes present in the Eucharist.
Jesus Himself stated when taking the bread, "This is My Body", then taking the wine saying, "This is My Blood".
Saint Paul also made clear in his first letter to the Corinthians that when referring to the bread and wine of the Eucharist he addressed it as the Body and Blood of Jesus.
The term, "Real Presence" refers to Christ's representation in the bread and wine that have been converted into Jesus Christ's Body and Blood.