guarantee letter; letter of guarantee
A formal written document that guarantees something, such as payment, product quality, or contractual obligations. Used in business, legal, and official contexts.
I submitted a letter of guarantee to the bank to receive a loan.
This product comes with a manufacturer's guarantee letter.
Compound of 保証 (guarantee) and 状 (document, letter). The exact historical derivation is uncertain, but the word follows a common pattern for formal documents in Japanese.