Derived from the Greek word for "circle, " an encyclical is among the church's most authoritative teaching documents.

CNN: Pope: 'Revolution' needed to fight climate change

Francis' bracing manifesto came Thursday in the form of an encyclical, a letter traditionally addressed from St.

CNN: Pope: 'Revolution' needed to fight climate change

Benedict had been widely expected to issue his fourth encyclical, concerning faith, before Easter.

NPR: Vatican Sends Message: Pope's Retirement For Real

In 1964, Pope Paul VI issued his first encyclical, "Ecclesiam Suam, " which stated his willingness to mediate in international disputes.

CNN: Sunday,

In 1979, Pope John Paul II published his first encyclical, "Redemptor Hominis, " in which he warned of the growing gap between rich and poor.

CNN: Saturday, March 15, 1997R

Lombardi said the encyclical would not be ready before Benedict's retirement.

NPR: Vatican Sends Message: Pope's Retirement For Real

Pope Benedict XIV promulgated an encyclical Vix Pervenit: On Usury and Other Dishonest Profit as late as 1745, and it has yet to be formally retracted.

FORBES: A Distant Mirror

In 1968, Pope Paul VI, in an encyclical entitled "Humanae Vitae" (Of Human Life), declared that any artificial forms of birth control were prohibited by the Roman Catholic Church.

CNN: Wednesday,

The CDF demanded that Dr Byrne make a public statement supporting the Humanae Vitae, the Papal encyclical on birth control, and the church's opposition to the ordination of women.

BBC: A nun on the run from Rome

In 1938, as he was growing even more concerned over what was happening in Germany and the rest of Europe, Pius XI asked an American Jesuit who was known for his anti-racist views to prepare a draft encyclical directly condemning racism and anti-racism and anti-Semitism.

NPR: Vatican Opens Pre-WWII Papal Archive