“Transubstantiation” According to Rome

Catechism of the Catholic Church, #1365: 
Because it is the memorial of Christ’s Passover, the Eucharist is also a sacrifice. The sacrificial character of the Eucharist is manifested in the very words of institution: “This is my body which is given for you” and “This cup which is poured out for you is the New Covenant in my blood.” In the Eucharist Christ gives us the very body which he gave up for us on the cross, the very blood which he “poured out for many for the forgiveness of sins.” 
(Catechism of the Catholic Church: With Modifications From The Editio Typica, [New York: Doubleday, 1997], #1365, p. 380.) See also: vatican.va.

Catechism of the Catholic Church, #1374: 

The mode of Christ’s presence under the Eucharistic species is unique. It raises the Eucharist above all the sacraments as “the perfection of the spiritual life and the end to which all the sacraments tend.” In the most blessed sacrament of the Eucharist “the body and blood, together with the soul and divinity, of our Lord Jesus Christ and, therefore, the whole Christ is truly, really, and substantially contained.” “This presence is called ‘real’—by which is not intended to exclude the other types of presence as if they could not be ‘real’ too, but because it is presence in the fullest sense: that is to say, it is a substantial presence by which Christ, God and man, makes himself wholly and entirely present.” 

(Catechism of the Catholic Church: With Modifications From The Editio Typica, [New York: Doubleday, 1997], #1374, pp. 383-384.) See also: vatican.va.

Catechism of the Catholic Church, #1375: 

It is by the conversion of the bread and wine into Christ’s body and blood that Christ becomes present in this sacrament. The Church Fathers strongly affirmed the faith of the Church in the efficacy of the Word of Christ and of the action of the Holy Spirit to bring about this conversion. (Catechism of the Catholic Church: With Modifications From The Editio Typica, [New York: Doubleday, 1997], #1375, p. 384.) See also: vatican.va.

Catechism of the Catholic Church, #1376: 

The Council of Trent summarizes the Catholic faith by declaring: “Because Christ our Redeemer said that it was truly his body that he was offering under the species of bread, it has always been the conviction of the Church of God, and this holy Council now declares again, that by the consecration of the bread and wine there takes place a change 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. This change the holy Catholic Church has fittingly and properly called transubstantiation.” 

(Catechism of the Catholic Church: With Modifications From The Editio Typica, [New York: Doubleday, 1997], #1376, pp. 384-385.) See also: vatican.va.

Catechism of the Catholic Church, #1377: 

The Eucharistic presence of Christ begins at the moment of the consecration and endures as long as the Eucharistic species subsist. Christ is present whole and entire in each of the species and whole and entire in each of their parts, in such a way that the breaking of the bread does not divide Christ. 

(Catechism of the Catholic Church: With Modifications From The Editio Typica, [New York: Doubleday, 1997], #1377, p. 385.) See also: vatican.va.

Catechism of the Catholic Church, #1378: 

Worship of the Eucharist. In the liturgy of the Mass we express our faith in the real presence of Christ under the species of bread and wine by, among other ways, genuflecting or bowing deeply as a sign of adoration of the Lord. “The Catholic Church has always offered and still offers to the sacrament of the Eucharist the cult of adoration, not only during Mass, but also outside of it, reserving the consecrated hosts with the utmost care, exposing them to the solemn veneration of the faithful, and carrying them in procession.” 

(Catechism of the Catholic Church: With Modifications From The Editio Typica, [New York: Doubleday, 1997], #1378, p. 385.) See also: vatican.va.

Fundamentals of Catholic Dogma: 

Christ becomes present in the Sacrament of the Altar by the transformation of the whole substance of the bread into His Body and of the whole substance of the wine into His blood. …This transformation is called Transubstantiation… 

(Ludwig Ott, Fundamentals of Catholic Dogma, ed. James Canon Bastible, trans. Patrick Lynch, [St. Louis: B. Herder Book Company, 1954], p. 377. Ecclesiastical approbation: Nihil Obstat: Jeremiah J. O’Sullivan D.D., Censor Deputatus. Imprimatur: Cornelius, Ep. Corgagiensis et Ap. Adm. Rossensis. 7 October, 1954.)

The Catholic Encyclopedia: 

First of all the whole structure of the discourse of promise demands a literal interpretation of the words: “eat the flesh of the Son of man, and drink his blood”. …it can be none other than His true Flesh and Blood, to be really eaten and drunk in Holy Communion. This is why Christ was so ready to use the realistic expression “to chew” (John vi, 54, 56, 58: τρώγειν) when speaking of this, His Bread of Life, in addition to the phrase, “to eat” (John vi, 51, 53: φαγεῖν). ...The impossibility of a figurative interpretation is brought home more forcibly by an analysis of the following text: “Except you eat the flesh of the Son of man, and drink his blood, you shall not have life in you. He that eateth my flesh and drinketh my blood, hath everlasting life: and I will raise him up in the last day. For my flesh is meat indeed: and my blood is drink indeed” (John vi, 54-56). ...Consequently, eating and drinking are to be understood of the actual partaking of Christ in person, hence literally. 

(Maurice M. Hassett, M., S.T.D., “Eucharist;” In: The Catholic Encyclopedia: Special Edition: Volume V, eds. Charles G. Herbermann, et al., [New York: The Encyclopedia Press, Inc., 1913.], pp. 573-574. Ecclesiastical approbation: Nihil Obstat, May 1, 1909, Remy Lafort, S.T.D., Censor. Imprimatur, John Cardinal Farley, Archbishop of New York.) See also: newadvent.org.

The United States Conference of Catholic Bishops: 

4. Does the bread cease to be bread and the wine cease to be wine?

Yes. In order for the whole Christ to be present—body, blood, soul, and divinity—the bread and wine cannot remain, but must give way so that his glorified Body and Blood may be present. Thus in the Eucharist the bread ceases to be bread in substance, and becomes the Body of Christ, while the wine ceases to be wine in substance, and becomes the Blood of Christ. As St. Thomas Aquinas observed, Christ is not quoted as saying, “This bread is my body,” but “This is my body” (Summa Theologiae, III q. 78, a. 5). 

(United States Conference of Catholic Bishops, The Real Presence of Jesus Christ in the Sacrament of the Eucharist: Basic Questions & Answers, [Washington, D.C.: United States Conference of Catholic Bishops, Inc., 2001], Q. 4, p. 8.) See also: usccb.org.

καὶ αὐτός ἐστιν πρὸ πάντων καὶ τὰ πάντα ἐν αὐτῷ συνέστηκεν ~ Soli Deo Gloria