What was the medieval “papal bull”?

The story behind the answer

In medieval usage, a “bull” was a solemn letter issued under the pope’s highest authority, employed for matters like defining doctrine, appointing bishops, excommunicating rulers, granting privileges, or calling crusades. It stood above routine correspondence and carried legal and spiritual weight across Christendom.

The name comes from the bulla, the lead seal attached to authenticate it—often depicting Saints Peter and Paul on one side and the reigning pope’s name on the other. That seal sometimes causes confusion, but it was the document that mattered. Famous examples include Unam Sanctam (1302), asserting papal supremacy, and Inter caetera (1493), which shaped European claims in the Atlantic world. Over time, less formal papal “briefs” emerged, but bulls remained the format for the most solemn acts.