A bulla was originally a circular plate or boss of metal, so called from its resemblance in form to a bubble floating upon water (Latin bullire, to boil). In the course of time the term came to be applied to the leaden seals with which papal and royal documents were authenticated in the early Middle Ages, and by a further development, the name, from designating the seal, was eventually attached to the document itself.From top:
Papal bull of Johannes XXII, Avignon, June 24, 1321.
Lead papal seal (bulla) dated from 1447 - 1455. Peter and Paul are depicted on one side, while the other has a Latin abbreviation for Pope Nicholas V.
Papal Bull of Pope Urban VIII, 1637.