An encyclical was originally a circular letter sent to all the churches of a particular area in the ancient Roman Church. At that time, the word could be used for a letter sent out by any bishop. The word comes from the Late Latin encyclios (originally from the Latin encyclius, a Latinization of Greek (enkyklios), meaning "circular", "in a circle", or "all-round", also part of the origin of the word encyclopedia).
Although the term "encyclical" originally simply meant a circulating letter, it acquired a more specific meaning within the context of the Catholic Church. In 1740, Pope Benedict XIV wrote a letter titled Ubi primum, which is generally regarded as the first encyclical. The term is now used almost exclusively for a kind of letter sent out by the Pope.
For the modern Roman Catholic Church, a papal encyclical is a specific category of papal document, a kind of pastoral letter concerning Catholic doctrine, sent by the Pope and usually addressed especially to patriarchs, primates, archbishops and bishops who are in communion with the Holy See. The form of the address can vary widely, and may concern bishops in a particular area, or designate a wider audience. Papal encyclicals usually take the form of a papal brief due to their more personal nature as opposed to the formal papal bull. They are usually written in Latin and, like most papal documents, the title of the encyclical is usually taken from its first few words (its incipit).
In the encyclical Humani generis, Pope Pius XII held that papal encyclicals, even when they are of ordinary magisterium, can nonetheless be sufficiently authoritative to end theological debate on a particular question:
It is not to be thought that what is set down in Encyclical letters does not demand assent in itself, because in this the popes do not exercise the supreme power of their magisterium. For these matters are taught by the ordinary magisterium, regarding which the following is pertinent: "He who heareth you, heareth Me." (Luke 10:16); and usually what is set forth and inculcated in Encyclical Letters, already pertains to Catholic doctrine. But if the Supreme Pontiffs in their acts, after due consideration, express an opinion on a hitherto controversial matter, it is clear to all that this matter, according to the mind and will of the same Pontiffs, cannot any longer be considered a question of free discussion among theologians.
Pope Pius XII issued ten encyclicals, mostly after 1945, three of them protesting the Soviet invasion of Hungary in order to crack down on the Hungarian Revolution in 1956: Datis nuperrime, Sertum laetitiae and Luctuosissimi eventus.