'Seal' (the stamp) is Latin 'sigillum' — a diminutive of 'signum' (sign). Literally 'a little sign.'
A device or substance used to close or fasten something securely; an embossed design stamped on wax to authenticate a document; to close or fasten securely; to fix a piece of wax bearing a design to a document to authenticate it.
From Old French 'seel' (a seal, a signet, an official stamp), from Vulgar Latin '*sigellum,' an altered form of classical Latin 'sigillum' (a small figure, a seal, a statuette, a signet), the diminutive of 'signum' (a mark, a sign, an image, a standard). 'Signum' derives from PIE *sekw- (to follow) through the sense of a mark that one follows — a sign that directs attention. A seal is literally 'a little sign' — the impressed mark that authenticates a document by carrying the device of a known authority
The Great Seal of the United States — featuring the bald eagle on one side and the unfinished pyramid with the Eye of Providence on the other — is the official emblem used to authenticate certain government documents. The term 'seal' in this context is etymologically identical to 'sign': the Great Seal is the Great Sign, the mark that authenticates the nation's most important acts. The medieval practice of sealing documents with wax impressed by a signet