Pope Pius IX Mariology

teaching that Mary was conceived free from original sin

Mediatrix is a title given to Mary, mother of Jesus used by some Christians. It refers to the role of the Blessed Virgin Mary as a mediator by intercession in the salvific redemption by her son Jesus Christ, the one proper Mediator by action. Mediatrix is an ancient title that has been used by many saints since at least the 5th century. Its use grew during the Middle Ages and reached its height in the writings of Louis de Montfort and Alphonsus Liguori in the 18th century.

1854 papal bull by Pope Pius IX that defines the Immaculate Conception of Mary as dogma

papal encyclical on the Immaculate Conception