The term magisterium is based on the Latin word for “teacher” (magister). In contemporary Catholic usage, it has several meanings.
First, it refers to the teaching authority which Christ has given to the Church. Here the term refers to the authority itself, not those who exercise it. This usage appears in statements like, “The Church exercises its magisterium when it authoritatively proclaims Christ’s teachings.”
Second, the term refers to those who exercise this teaching authority—in other words, to the pope and the bishops teaching in union with him. Collectively, they are referred to as the “Magisterium,” as in “the Magisterium has infallibly taught that God is a Trinity.”
Third, the term can refer to a particular body of teachings that have been authoritatively proclaimed.