Despite what you may think of Roman Catholic
doctrine, the term papal bull
actually refers to a solemn edict issued by the pope
to grant privileges or make instructions known.
The name "bull" comes from the bulla, a special lead seal that authenticated the document an official. At first the seal had only a written inscription reading "PAPA" on one side and "LEONIS" on the other, but later seals depicted a picture of the heads of Saints Peter and Paul. Papal bulls were always delivered open, with the seals attached to the bottom of the single sheet that was written on only on one side.
Papal bulls descended from Roman Imperial documents, and thus were written in the Roman style on papyrus, until the 11th century, when parchment began to be used. Over the centuries an elaborate set of symbolic formatting developed for the bulls, which had special, carefully defined methods for recording the date and the pope's name and other information.