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 ἐγκύκλιος (enkyklios), meaning "circular", "in a circle", or "all-round", also part of the origin of the word encyclopedia).
The term is now primarily associated with papal encyclicals from the Catholic Church for letters outlining the church's perspective on a topic, but is also used among Anglicans and the Eastern Orthodox Church.