Encyclical

Encyclical, letter from the Pope to the bishops of the Roman Catholic Church. Papal encyclicals set out guidelines for the application of theological and social Church teachings. Among the best-known encyclicals are Rerum Novarum (1891), on the condition of the working classes, by Leo XIII; Pacem in Terris (1963), on relationships between the Church and state, by John XXIII; and the controversial Humanae Vitae (1968), on birth control, by Paul VI.