Magisterium (from the Latin magister, "teacher") is a technical ecclesiastical term in Catholicism referring to the teaching ability and authority of the Pope and those Bishops who are in union with him. According to Catholic doctrine, the Magisterium is able to teach or interpret the truths of the Faith infallibly.
"The task of interpreting the Word of God authentically has been entrusted solely to the Magisterium of the Church, that is, to the Pope and to the bishops in communion with him." (Catechism of the Catholic Church, 2nd ed. [1997], pt. 1, sect. 1, ch. 2, art. 2, III [#100]).
Roman Catholic theologians divide the functions of the magisterium into the extraordinary magisterium and the ordinary magisterium. The extraordinary magisterium includes the teachings of a pope when he is speaking ex cathedra, and the solemn dogmatic or moral definitions and doctrinal condemnations of an ecumenical council (traditionally expressed in conciliar canons and decrees). The ordinary magisterium includes the group of papal and episcopal teachings which are not ratified by a solemn definition, even if these teachings take place within the context of an ecumenical council. (Catholic theologians and ecclesiastics generally agree that some councils, notably Vatican II, made no such solemn judgments.)
Both ordinary and extraordinary functions of the magisterium are considered infallible: "Wherefore, by divine and Catholic faith all those things are to be believed which are contained in the word of God as found in Scripture and tradition, and which are proposed by the Church as matters to be believed as divinely revealed, whether by her solemn judgment or in her ordinary and universal Magisterium." (First Vatican Council, Dei Filius 8.) However, the criteria for the infallibility of these two functions of the magisterium are different. See Infallibility of the Church and papal infallibility for more details.
Magisterial teaching is derived from Sacred Tradition and Sacred Scripture (i.e., the Bible). The belief that God reveals his teachings to humanity via infallible Sacred Tradition and Sacred Scripture is one basis for infallibility; another basis is that the Holy Spirit guides the Magisterium. Catholics are obliged to believe all that is taught infallibly by the Magisterium with the assent of faith (fides divina), i.e. with the fullness of their faith. Catholics are, in general, also obliged to believe the non-infallible teachings of the Magisterium, but with a different type or degree of assent, called religious assent (fides ecclesiastica).
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