A Papal bull is a particular type of letters patent or charter issued by a pope. It is named after the bulla that was appended to the end to authenticate it.
Papal bulls were originally issued by the pope for many kinds of communication of a public nature, but after the fifteenth century, only for the most formal or solemn of occasions. Modern scholars have retroactively used the term "Bull" to describe any elaborate papal document issued in the form of a decree or privilege (solemn or simple), and to some less elaborate ones issued in the form of a letter. Popularly, the name is used for any papal document that contains a metal seal.
Papal bulls have been in use at least since the sixth century, but the term was not first used until around the middle of the thirteenth century and then only for internal un-official papal record keeping purposes; the term had become official by the fifteenth century, when one of the offices of the Papal chancery was named the "register of bulls" (registrum bullarum).
Today, the bull is the only written communication in which the Pope will refer to himself as episcopus servus servorum Dei, meaning "Bishop, Servant of the Servants of God." For instance, Benedict XVI, when he issues a decree in bull form, will begin the document with Benedictus, Episcopus, Servus Servorum Dei. While it used to always bear a metal seal, it now does so only on the most solemn occasions. It is today the most formal type of letters patent issued by the Vatican Chancery in the name of the Pope.
The bull's format began with one line in tall elongated letters containing three elements: the Pope's name, the Papal title episcopus servus servorum Dei, meaning 'bishop, servant of the servants of God', and the few Latin words that constituted the incipit from which the bull would also take its name for record keeping purposes, but which might not be directly indicative of the bull's purpose.
The body of the text had no specific conventions for its formatting; it was often very simple in layout. The closing section consisted of a short datum, mentioning the place it was issued, the day of the month and the year of the pope's pontificate and signatures, near which was attached the seal.
For the most solemn bulls, the Pope would sign the document himself, in which case he used the formula Ego N. Catholicae Ecclesiae Episcopus (I, N., Bishop of the Catholic Church). Following the signature in this case would be an elaborate monogram, the signatures of any witnesses, and then the seal. Nowadays, a member of the Roman Curia signs the document on behalf of the Pope, usually the Cardinal Secretary of State, and thus the monogram is omitted.
The most distinctive characteristic of a bull was the metal seal, which was usually made of lead, but on very solemn occasions was made of gold (as Byzantine imperial deeds often were). It depicted the founders of the Church of Rome, the apostlesPeter and Paul, identified by the letters Sanctus PAulus and Sanctus PEtrus. The name of the issuing pope is on the reverse side. This was then attached to the document either by cords of hemp (in the case of letters of justice, and executory) or by red and yellow silk (in the case of letters of grace) that was looped through slits in the vellum of the document. Bulla is the name of this seal, which to ancient observers looked like a bubble floating on water: Latin bullire, "to boil".
Since the late eighteenth century, the lead bulla has been replaced with a red ink stamp of Saints Peter and Paul with the reigning Pope's name encircling the picture, though very formal letters, e.g. the bull of Pope John XXIII convoking the Second Vatican Council, still receive the lead seal.
Original papal bulls exist in quantity only after the eleventh century onward when the transition from fragile papyrus to the more durable parchment was made. None survives in entirety from before 819. Some original leaden seals, however, still survive from as early as the 6th century.
In terms of content, the bull is simply the format in which a decree of the Pope appears. Any subject may be treated in a bull, and many were and are, including statutory decrees, episcopal appointments, dispensations, excommunications, apostolic constitutions, canonizations and convocations. The bull was the exclusive letter format from the Vatican until the fourteenth century, when the Papal brief began to appear. The Papal brief is the less formal form of papal communication and is authenticated with a wax impression (now a red ink impression) of the Ring of the Fisherman. There has never been an exact distinction of usage between a bull and a brief, but nowadays most letters, including Papal encyclicals, are issued as briefs.