surety

Short definition from Wiktionary

Noun

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