The name ‘bull’ comes from the Latin ‘bulla’ which refers to the seal that was attached at the bottom of a papal edict. The basic definition of a papal bull according to the Catholic encyclopedia is "an Apostolic letter with a leaden seal".
In the 16th century the bulls could be marriage dispensations, excommunications and in the case of Gregory XIII in 1582, a calendar reform. Some examples from Tudor History: The Bull of Pope Innocent VIII approving the marriage of Henry VII and Elizabeth of York, the two issued by Julius II that granted the dispensation allowing Henry VIII to marry his brother Arthur’s widow, Catherine of Aragon, and the bull by Pius V against Elizabeth I. The bull against Elizabeth declared her a heretic and released her Catholic subjects from their duty to be loyal to their Queen. In 1588 Sixtus V issued another bull against Elizabeth upholding her excommunication and that supported the Spanish Armada’s actions against England.