guarantee
/ˌɡæɹənˈtiː/
Anything that assures a certain outcome.
Can you give me a guarantee that he will be fit for the match?
A legal assurance of something, e.g. a security for the fulfillment of an obligation.
More specifically, a written declaration that a certain product will be fit for a purpose and work correctly; a warranty
The cooker comes with a five-year guarantee.
The person to whom a guarantee is made.
A person who gives such a guarantee; a guarantor.