During the past 200 years, encyclicals have become an essential part of the papacy. These letters have given popes the opportunity to address urgent concerns in the Church and the world.
Prior to that there may have been “encyclical” letters, letters from the pope addressed to bishops of a particular region, but the more formal type of letter didn’t come into existence until 1740.
It began with the pontificate of Benedict XIV, who wrote Epistola encyclica et commonitoria, a letter on the duties of the episcopal office. After that, each pope used this official type of letter in various ways.