guarantee
Definitions
A formal promise or assurance, typically in writing, that certain conditions will be fulfilled or that a product is of a specified quality.
To provide a formal assurance or promise, especially that something will be done or that a product will be repaired or replaced if it fails.
To make a certain outcome or result inevitable or sure to happen.
Examples
The company offers a money-back guarantee if you are not satisfied with the product.
Hard work does not always guarantee success, but it certainly increases your chances.