Long before the term Transubstantiation came into being there was the unquestioned belief that Christ Body and Blood were really present under the appearances of bread and wine in the Sacrifice of Mass.
In (c. 165 AD) St. Justin, martyr, stated it thus: "We do not consume the Eucharist bread and wine as if it were ordinary food and drink, for we have been taught that as Jesus Christ our Savior became a man of flesh and blood by the power of the Word of God, so also the food that our flesh and blood assimilates for its nourishment becomes the flesh and blood of the incarnate Jesus by the power of his own words contained in the prayer of thanksgiving. (First Apology of St. Justin Martyr (Cap. 66-67: PG. 6, 427-431) Likewise, the Eastern Church Fathers before the 6th Century used the expression metaousiosis meaning "change of being" which is essentially the same idea as Transubstantiation
The first serious controversy regarding the nature of the Eucharist arose in the 9th Century. Renatus, a monk of the French Abbey of Corbie, held that Christ's body in the Eucharist could not be the same as Christ's historical body once on earth and now in heaven because the Eucharistic body is invisible, impassible, and spiritual. Renatus wanted to hold on to the Real Presence but stressed the Eucharist as symbolic rather than corporal. (Jon A. Hardon, S.J., The Catholic Catechism, Doubleday & Co., Inc., Garden City, NY, 1975, p. 461)
Martin Luther (14831546) was a German monk who believed in the Real Presence of Christ along with the existence of real bread and wine, in other words, Christ presence permeated the bread and wine but did not change it. The position if known as Con-substantiation. Another position suggested by Gitmund of Aversa says that Christ and the substance of bread are united by a Hypostatic Union.
The Council of Trent
(1545-1543) defined what is to be believed as follows:
"...Christ, our Redeemer, declared that which He offered under the species of bread to be truly His own body, therefore this has ever been a firm belief in the Church of God, and this holy Synod doth now declare it anew, that, by the consecration of the bread and of the wine, a conversion is made of the whole substance of the bread into the substance of the body of Christ our Lord, and of the whole substance of the wine into the substance of His blood; which conversion is, by the holy Catholic Church, suitably and properly called Transubstantiation." "...[This] holy Synod teaches, and openly and simply professes, that, in the august sacrament of the holy Eucharist, after the consecration of the bread and wine, our Lord Jesus Christ, true God and man, is truly, really, and substantially contained under the species of those sensible things."(Council of Trent, 13th Session, Ch. 4,1 On Transubstantiation)
The term Transubstantiation was coined by the theologians of the 12th Century. As used to define the Eucharist. "Trans" means "change", that is, change of substance. In philosophy substance is defined as that which makes a thing be what it is, and as such is invisible. When Jesus changed water into wine at Canna, this was a change of substance which also changed its sensible aspects. Wine like water is a liquid but has a different color and taste than water. In the case of the Eucharist, the reality of bread is changed into Christ presence, the whole Christ, body, soul and divinity, while the appearances of bread and wine remain. Why? Obviously, if they did not remain, we would have no way of apprehending Christ's presence through our senses. St. Thomas Aquinas also notes that the sacrament of the Eucharist has the appearance of food and drink because it does for the soul what bread and wine does for the body: it sustains, builds up and restores.
Transubstantiation becomes more understandable if we keep in mind that God created and sustains the universe out of nothing and that He elevated matter to life in ever increasing changes of life forms, that is, substantial changes that give creatures their unique identity. Roy Abraham Varghese calls this process "Metamorphic Creation". He states, "Only an awareness of the mystery of the entire and constant dependence of each object on God both for its existence and its existence with the kind of physical substrate and properties that it has will prepare our minds for the greater mystery of Transubstantiation."