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A bulla was originally a circular plate or boss of metal, so called from its resemblance in <!--stripped-->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 <!--qr49-->seals with which papal and royal documents were authenticated in the early Middle Ages, and by a further development, the name, from designating the <!--qr49-->seal, was eventually attached to the document itself. This did not happen before the thirteenth century and the name bull was only a popular term used almost promiscuously for all kinds of instruments which issued from the papal <!--qr79-->chancery. A much more precise <!--qr79-->acceptance has prevailed since the fifteenth century, and a <!--qr69--><!--stripped-->bull has long stood in sharp contrast with <!--qr66--><!--stripped-->certain other <!--stripped-->forms of papal documents. For practical purposes a <!--qr69--><!--stripped-->bull may be conveniently defined to be "an <!--qr76-->Apostolic letter with a leaden <!--qr49-->seal," to which one may add that in its superscription the pope invariably takes the title of episcopus, servus servorum Dei. <!--qk 206-->

In official language papal documents have at all times been called by various names, more or less descriptive of their <!--qr49-->character. For example, there are "constitutions," i.e., decisions addressed to all the faithful and determining some <!--qr66--><!--stripped-->matter of faith or <!--qr79-->discipline; "encyclicals," which are letters sent to all the bishops of Christendom, or at least to all those in one particular country, and intended to guide them in their relations with their flocks; "decrees," pronouncements on points affecting the general welfare of the Church; "decretals" (epistolae decretales), which are papal replies to some particular difficulty submitted to the Holy See, but having the force of precedents to rule on all analogous cases. "Rescript," again, is a <!--stripped-->form applicable to almost any <!--stripped-->form of <!--qr76-->Apostolic letter which has been elicited by some previous <!--stripped-->appeal, while the <!--qr66-->nature of a "privilege" speaks for itself. But all these, down to the fifteenth century, seem to have been expedited by the papal <!--qr79-->chancery in the shape of <!--qr69--><!--stripped-->bulls authenticated with leaden <!--qr49-->seals, and it is common enough to apply the term bull even to those very early papal letters of which we know little more than the <!--qr66-->substance, independently of the <!--stripped-->forms under which they were issued.

It will probably be most convenient to divide the subject into periods, noting the more characteristic features of <!--stripped-->papal documents in each age.