guarantee

noun

1. Anything that assures a certain outcome.

   “Can you give me a guarantee that he will be fit for the match?”

   “After his actions were challenged by foreign governments and Parliament initially tried to put a stop to his action, Denman returned home and argued his case with enough force that, by 1848, the Royal Navy was handed active permission and encouragement to raze every last slave factory they could find to the ground, and full authority to stop any ship, of any flag, that was thought to be a slaver, with a guarantee with^([sic]) no censure from the government.”

2. A legal assurance of something, e.g. a security for the fulfillment of an obligation.
3. (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.”

4. The person to whom a guarantee is made.
5. (colloquial)A person who gives such a guarantee; a guarantor.

   “But God who is the great Guarantee for the Peace , Order , and good behaviour of Mankind”

verb

1. To give an assurance that something will be done right.
2. To assume or take responsibility for a debt or other obligation.
3. To make something certain.

   “The long sunny days guarantee a good crop.”