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).The term has been used by Catholics, Anglicans and the Eastern Orthodox.

The Catholic Church, also known as the Roman Catholic Church, is the largest Christian church, with 1.3 billion baptised Catholics worldwide as of 2019. As the world's oldest and largest continuously functioning international institution, it has played a prominent role in the history and development of Western civilisation. The church consists of 24 particular churches and almost 3,500 dioceses and eparchies around the world. The pope, who is the bishop of Rome, is the chief pastor of the church. The bishopric of Rome, known as the Holy See, is the central governing authority of the church. The administrative body of the Holy See, the Roman Curia, has its principal offices in Vatican City, a small enclave of Rome, of which the pope is head of state.

The core beliefs of Catholicism are found in the Nicene Creed. The Catholic Church teaches that it is the one, holy, catholic and apostolic church founded by Jesus Christ in his Great Commission, that its bishops are the successors of Christ's apostles, and that the pope is the successor to Saint Peter, upon whom primacy was conferred by Jesus Christ. It maintains that it practises the original Christian faith taught by the apostles, preserving the faith infallibly through scripture and sacred tradition as authentically interpreted through the magisterium of the church. The Latin Church, the twenty-three Eastern Catholic Churches, and institutes such as mendicant orders, enclosed monastic orders and third orders reflect a variety of theological and spiritual emphases in the church.Of its seven sacraments, the Eucharist is the principal one, celebrated liturgically in the Mass. The church teaches that through consecration by a priest, the sacrificial bread and wine become the body and blood of Christ. The Virgin Mary is venerated as the Perpetual Virgin, Mother of God, and Queen of Heaven; she is honoured in dogmas and devotions. Catholic social teaching emphasises voluntary support for the sick, the poor, and the afflicted through the corporal and spiritual works of mercy. The Catholic Church operates thousands of Catholic schools, hospitals, and orphanages around the world, and is the largest non-government provider of education and health care in the world. Among its other social services are numerous charitable and humanitarian organisations.

The Catholic Church has profoundly influenced Western philosophy, culture, art, music and science. Catholics live all over the world through missions, diaspora, and conversions. Since the 20th century, the majority have resided in the southern hemisphere, partially due to secularisation in Europe and increased persecution in the Middle East. The Catholic Church shared communion with the Eastern Orthodox Church until the East–West Schism in 1054, disputing particularly the authority of the pope. Before the Council of Ephesus in AD 431, the Church of the East also shared in this communion, as did the Oriental Orthodox Churches before the Council of Chalcedon in AD 451; all separated primarily over differences in Christology. In the 16th century, the Reformation led to Protestantism also breaking away. From the late 20th century, the Catholic Church has been criticised for its teachings on sexuality, its doctrine against ordaining women, and its handling of sexual abuse cases involving clergy.