* What is the MAGISTERIUM? The Church's teaching authority, vested in the bishops, as successors of the Apostles, under the Roman Pontiff, as successor of St. Peter. Also vested in the Pope, as Vicar of Christ and visible head of the Catholic Church. (Etym. Latin magister, master.)

MAGISTERIUM, EXTRAORDINARY. The Church's teaching office exercised in a solemn way, as in formal declarations of the Pope or of ecumenical councils of bishops approved by the Pope. When the extraordinary magisterium takes the form of papal definitions or conciliar decisions binding on the consciences of all the faithful in matters of faith and morals, it is infallible.

MAGISTERIUM, ORDINARY. The teaching office of the hierarchy under the Pope, exercised normally, that is, through the regular means of instructing the faithful. These means are all the usual channels of communication, whether written, spoken, or practical. When the ordinary magisterium is also universal, that is, collectively intended for all the faithful, it is also infallible.

Source: Pocket Catholic Dictionary - John A. Hardon, S.J.
            Abridged Edition of the Modern Catholic Dictionary
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