A Surety

1. Certainty. https://en.wikipedia.org/wiki/Surety
2. That which makes sure; that which confirms; ground of confidence or security.
3. (law) A promise to pay a sum of money in the event that another person fails to fulfill an obligation.
4. (law) One who undertakes to pay money or perform other acts in the event that his principal fails therein.
5. A substitute; a hostage.
6. Evidence; confirmation; warrant.