A 'bull' refers to any document that is affixed with a seal of authority. The word comes from the Latin word for seal, bulla (pl. bullae).
Seals have been used for this purpose since antiquity. Neh 9:38 reports the making of an agreement with the leaders, Levites and priests affixing their seals to it." Rev 5 reports John's vision of "a scroll with writing on both sides and sealed with seven seals."
In the excavation of part of the "City of David" in Jerusalem, archaeologists have recovered a trove of burnt clay bullae bearing the seal of Baruch son of Neriah, Jeremiah's secretary.
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