Co-Redemptrix is a title used by some Roman Catholics for the Blessed Virgin Mary, as well as a Catholic theological concept referring to Mary's role in the redemption of man. It has always been controversial and has never formed part of the dogma of the Church. The term Co-Redemptrix refers to a subordinate but essential participation by the Blessed Virgin Mary in redemption, notably that she gave free consent to give life to the Redeemer, to share his life, to suffer with him under the cross, to offer his sacrifice to God the Father for the sake of the redemption of mankind. Related to this belief is the concept of Mary as Mediatrix, which is a separate concept but regularly included by faithful who use the title Co-Redemptrix.
The concept of co-redemption is not new. Even before the year 200, the Church Father Irenaeus referred to Mary as the cause of our salvation (Latin: causa salutis) given her "fiat". It is a concept which was the subject of considerable theological debate, reaching a peak in the 15th century. The concept was especially common in the late Middle Ages, when it was promoted heavily among the Franciscans, and often resisted by the Dominicans.
By the early 16th century the hopes of the concept becoming Catholic doctrine had receded, and have never seriously revived. In more recent times, the title has received some support from the Catholic Magisterium though it is not included in the concluding chapter of the apostolic constitution Lumen gentium of the Second Vatican Council, which chapter many theologians hold to be a comprehensive summary of Roman Catholic Mariology. Some, in particular the adherents of the Amsterdam visions, have petitioned for a dogmatic definition, along with Mediatrix, but recent high-level comments in the Church have not encouraged these hopes.