The Immaculate Conception is a dogma of the Catholic Church maintaining that from the moment when she was conceived the Blessed Virgin Mary was kept free of original sin and was filled with the sanctifying grace normally conferred during baptism. It is one of the four dogmas in Roman Catholic Mariology. Mary is often called the Immaculata (the Immaculate One), particularly in artistic and cultural contexts.
The Immaculate Conception should not be confused with the perpetual virginity of Mary or the virgin birth of Jesus; it refers to the conception of Mary by her mother, Saint Anne. Although the belief was widely held since at least Late Antiquity, the doctrine was not formally proclaimed until December 8, 1854, by Pope Pius IX in his papal bull Ineffabilis Deus. It is not formal doctrine except in the Roman Catholic Church. The Feast of the Immaculate Conception is observed on December 8 in many Catholic countries as a holy day of obligation or patronal feast, and in some as a national public holiday.
Pope Pius IX defines the dogma: “To the honor of the holy and undivided Trinity, to the glory and adornment of the Virgin Mother of God, to the exaltation of the Catholic faith, and the increase of the Catholic religion, We, by the authority of Jesus Christ, Our Lord, of the Blessed Apostles, Peter and Paul and by Our Own, declare, pronounce, and define that the doctrine which holds that the Blessed Virgin Mary, at first instant of her conception, by a singular privilege and grace of the omnipotent God, in consideration of the merits of Jesus Christ, the Savior of mankind was preserved free from all stain of original sin, has been revealed by God, and therefore is to be firmly and constantly believed by all the faithful.”
The doctrine of the Immaculate Conception is synthesized in the statement: “The Blessed Virgin Mary, at the first instant of her conception, by a singular privilege and grace of the omnipotent God, in consideration of the merits of Jesus Christ, the Savior of mankind was preserved from all stain of original sin.” It affirms: (1) this immunity was a special grace from God, (2) through the foreseen merits of Christ, (3) Mary was exempt from original sin contracted by the rest of mankind, and (4) the exemption took place at the first moment of her conception in the womb of her mother.
Sources: The Catholic Catechism: A Contemporary Catechism of the Teachings of the Catholic Church by John A. Hardon, S.J.