provide a formal assurance, especially that certain conditions will be fulfilled relating to a product, service, or transaction

the company guarantees to refund your money

promise with certainty

no one can guarantee a profit on stocks and shares

a formal assurance (typically in writing) that certain conditions will be fulfilled, especially that a product will be repaired or replaced if not of a specified quality

we offer a 10-year guarantee against rusting

the treaty provides a guarantee of free trade

an undertaking to answer for the payment or performance of another person's debt or obligation in the event of a default by the person primarily responsible for it