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a round metal seal made of lead, silver, or gold, which was usually used in the Middle Ages to fasten papal, imperial, and royal acts (documents) by being tied to the document by a string; the act itself, with the bulla tied to it, was also called a bulla. [In English the act is called a bull.] The most important royal and imperial decrees, fastened with golden seals, came to be called Golden Bulls (such as the Golden Bull of 1222, issued by the Hungarian King Andrew II, and the Golden Bull of 1356, issued by Charles IV of the Holy Roman Empire). In later centuries, up until modern times, the term “bulla” was used to designate only the most important acts issued by the popes of Rome.
The supplementary historical discipline known as sphragistics studies medieval bull seals.