From the Latin, magister, meaning teacher. The teaching office of the Church, and especially the teaching office as exercised by the Pope alone, speaking in a way that clearly binds the Universal Church to believe in what he is pronouncing, or by the Pope together with all the Catholic bishops in an ecumenical council issuing such binding pronouncements.
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, and it does so infallibly within the Sacred Magisterium.