an assurance or formal promise that something is of the specified quality and that it will be repaired or replaced in case it isn't of the promised quality;
e.g. They offered a 5-year guarantee on the blender that we bought.
(verb)
to give an assurance or formal promise that something is of the promised quality;
e.g. The salesman guaranteed me that the blender would not break or stop working for at least 4 years.
(verb)
to secure by giving or taking security;
e.g. At all hotels, a credit card guarantees your reservation.