A design impressed on wax or other plastic material, or the same adhering to, if attached by cords to a document as evidence of its authenticity.
The use of a seal by men of wealth and position was common before the Christian era.
Through the centuries the practise spread and at the synpd pf Chalons-sur-Saone in 818, it was enacted that letters under the bishop's seal should be given to priests when they lawfully quitted their own diocese.
The custom of bishops possessing seals may be assumed to have been rather general from this date.
At tirst the seal was used for securing the document, but later it was attached to its face and served as an authentication; the deed was thus held to be valid as long as the seal remained intact.
It soon followed that not only kings and bishops but every kind of body corporate, cathedral chapters, municipalities, monasteries, etc., used a seal to validate the acts executed in their name.
During the early Middle Ages lead seals, or "bulls" were used, but except in the case of the papal chancery, it became the universal practise to take the impressions in wax.
The importance of the seal necessitated that when authority passed into new hands the old seal should be destroyed and a new one made; when the pope dies it is the first duty of the Cardinal Camerlengo to break up the fisherman's ring, the papal seal.
New Catholic Dictionary