Extraordinary Form

"An extraordinary form of the Roman Rite" is a phrase used in Pope Benedict XVI's 2007 motu proprio Summorum Pontificum to describe the liturgy of the 1962 Roman Missal, widely referred to as the Tridentine Mass, and which is performed in Ecclesiastical Latin. The phrase distinguishes the liturgy of the Missal issued by Pope John XXIII in 1962 from that of the Missal revised by Pope Paul VI in 1969 (the "ordinary form"), which "obviously is and continues to be the normal Form – the Forma ordinaria – of the Eucharistic Liturgy".

In the first article of the motu proprio, Pope Benedict designated the 1962 form as "an extraordinary form of the Church's Liturgy" and as "an extraordinary expression of the same lex orandi of the Church", and in his accompanying letter to the bishops he said that the 1962 Missal "will now be able to be used as a Forma extraordinaria of the liturgical celebration".

The full text of Article 1 of the motu proprio is:

Art 1. The Roman Missal promulgated by Pope Paul VI is the ordinary expression of the lex orandi (rule of prayer) of the Catholic Church of the Latin rite. The Roman Missal promulgated by Saint Pius V and revised by Saint John XXIII is nonetheless to be considered an extraordinary expression of the same lex orandi of the Church and duly honoured for its venerable and ancient usage. These two expressions of the Church's lex orandi will in no way lead to a division in the Church's lex credendi (rule of faith); for they are two usages of the one Roman rite.

The context of the expression in the accompanying letter is as follows:

The last version of the Missale Romanum prior to the Council, which was published with the authority of Pope John XXIII in 1962 and used during the Council, will now be able to be used as a Forma extraordinaria of the liturgical celebration. It is not appropriate to speak of these two versions of the Roman Missal as if they were "two Rites". Rather, it is a matter of a twofold use of one and the same rite.
As for the use of the 1962 Missal as a Forma extraordinaria of the liturgy of the Mass, I would like to draw attention to the fact that this Missal was never juridically abrogated and, consequently, in principle, was always permitted. At the time of the introduction of the new Missal, it did not seem necessary to issue specific norms for the possible use of the earlier Missal.

...
Wikipedia