An encyclical is a pastoral letter written by the pope for the whole Roman Catholic Church on matters of doctrine, morals, or discipline. It is usually addressed to Catholic bishops, but some encyclicals, such as Pacem in terris (Latin: “Peace on Earth”) issued by John XXIII in 1963, have been addressed also to “the entire Catholic World” and to “all Men of Good Will.”
A social encyclical is one that addresses specific and current moral issues. These include Leo XIII’s Rerum novarum (“Of New Things”), which was issued in 1891 and laid the foundation of Catholic social teaching, and Francis’s Laudato si’ (“Praise Be to You”), which was published in 2015 and addressed climate change and care for the Earth.