noun - the study of seals from the Latin sigillum or seal

Also known as sphragistics (see the Webster 1913 definition) which is derived from the Greek for seal; a sub-discipline of the more wide-ranging field of Manuscript Studies.

A big deal in Byzantine history apparently, due to the large number of surviving examples (over 60,000), mostly in lead but also in gold (which was used for sealing imperial charters), the careful study of which can reveal a great deal about the structure of the imperial administration and ecclesiastical hierarchy.