On 8 December 1854, Pope Pius IX issued the apostolic constitution Ineffabilis Deus, formally defining as Catholic dogma that Mary was conceived without the stain of original sin, preserved from it from the very first moment of her own conception through a special grace granted in anticipation of the redemptive merits of her son, Jesus Christ. The doctrine addresses Mary's own conception by her parents, traditionally named Joachim and Anne in later Christian tradition though not named in the New Testament itself, and should be distinguished from the virginal conception of Jesus, a separate and older Christian teaching concerning a different event.
The teaching had circulated in Christian devotional and theological life for many centuries before its formal definition, debated at length among medieval theologians, including a notable disagreement between Franciscan theologians who generally supported the doctrine and Dominican theologians, including Thomas Aquinas, who expressed reservations about how it could be reconciled with the universal need for redemption through Christ that Catholic theology otherwise affirmed applied to every human being without exception.
Pius IX's 1854 definition settled the question authoritatively within Catholic teaching, resolving centuries of internal theological debate by formal papal pronouncement, issued before the doctrine of papal infallibility itself was formally defined at the First Vatican Council sixteen years later in 1870, though later Catholic theology generally treats the 1854 definition as an exercise of the same underlying authority.