Attwater on the Magisterium
MAGISTERIUM: (Lat. magister, a master). The Church's divinely appointed authority to teach the truths of religion, "Going therefore, teach ye all nations . . . teaching them to observe all things whatsoever I have commanded you" (Matt. xxviii, 19-20). This teaching is infallible: "And behold I am with you all days, even to the consummation of the world" (ibid.)
The solemn megisterium is that which is exercised only rarely by formal and authentic definitions of councils or popes. Its matter comprises dogmatic definitions of cumenical councils or of the popes teaching ex cathedra, or of particular councils, if their decrees are universally accepted or approved in solemn form by the pope; also creeds and professions of faith put forward or approved in solemn form by the pope or cumenical council.
The ordinary magisterium is continually exercised by the Church especially in her universal practices connected with faith or morals, in the unanimous consent of the Fathers (q.v.) and theologians, in the decisions of the Roman Congregations concerning faith and morals, in the common sense (q.v.) of the faithful, and various historical documents in which the faith is declared. All these are founts of teaching which as a whole is infallible. They have to be studied separately to determine how far and in what conditions each of them is an infallible source of the truth.
Donald Attwater, A Catholic Dictionary (NY: Macmillan, 1958), s.v. "Magisterium," p. 301 (paragraph spacing added) +Georgius L. Craven. Epus. Sebastopolis, Vic. Cap. Westmon., Westmonasterii, die 30 Januarii, 1957..