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 form of the address can vary widely and may concern bishops in a particular area, or designate a wider audience.
On social issues, Pope Leo XIII promulgated the encyclical Rerum novarum (1891), which was followed by Quadragesimo anno (1931) of Pius XI and Centesimus annus (1991) of John Paul II.
Pope Pius XII issued ten encyclicals, mostly after 1945, three of them protesting against the Soviet invasion of Hungary which suppressed the Hungarian Revolution in 1956: Datis nuperrime, Laetamur admodum and Luctuosissimi eventus.