A seal is an impression printed on, embossed upon, or affixed to a document (or any other object) in order to authenticate it, in lieu of or in addition to a signature. Only in the case of a dry seal the imprint is made as a relief resulting from the greater pressure on the paper where the high parts of the seal touch; in all other cases a liquid or liquified medium (such as ink or wax) is used, allowing the choice of another color than the paper's.
The word is also used as the generic term to describe the devices used to make such impressions, such as (signet) rings. Sigilliography is the term used for the study of seals.