Ineffabilis Deus — the definition — English translation as posted at papalencyclicals.net (public domain)
Wherefore, in humility and fasting, we unceasingly offered our private prayers as well as the public prayers of the Church to God the Father through his Son, that he would deign to direct and strengthen our mind by the power of the Holy Spirit. In like manner did we implore the help of the entire heavenly host as we ardently invoked the Paraclete. Accordingly, by the inspiration of the Holy Spirit, for the honor of the Holy and undivided Trinity, for the glory and adornment of the Virgin Mother of God, for the exaltation of the Catholic Faith, and for the furtherance of the Catholic religion, by the authority of Jesus Christ our Lord, of the Blessed Apostles Peter and Paul, and by our own: “We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful.”
Reproduced in full. Translation: English translation as posted at papalencyclicals.net (Public domain).
Why Pitre cites it
Mary 'preserved free from all stain of original sin… by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ' — which is why she can call God her Saviour.
Notes
The whole paragraph containing the solemn definition is given (it opens with the invocation of the Trinity and the apostolic authority). Footnote [29] removed. The 1854 Latin original is public domain; the English translation used here is the one hosted at papalencyclicals.net.