Refers specifically to the seal of the Emperor of Japan, used in official state documents. A formal, historical term.
印璽は天皇の御璽と国璽の総称である。
The imperial seal is a collective term for the Emperor's privy seal and the state seal.
Compound of 印 (seal, stamp) and 璽 (imperial seal). The exact historical derivation is uncertain, but the term has been used in Japanese court and state contexts for centuries.