A pastoral letter on an important matter addressed by the pope to the whole Church and even to the whole world. Among the most famous encyclicals have been Pope Leo XIII's Rerum Novarum and Pope John XXIII's Pacem in Terris.
An encyclical was a circular letter sent to all the churches of a particular area in the ancient Christian church. At that time, the word could be used of a letter sent out by any bishop. The word comes from Latin encyclia (from the Greek "en kyklo, ") meaning "general" or "encircling", which is also the origin of the word "encyclopedia".