Guarantee means to promise that something will definitely happen or that something is certainly true. When a store guarantees that a product will work, they're making a firm promise that if it breaks, they'll fix or replace it. When your friend guarantees they'll show up to your birthday party, they're saying you can count on them absolutely.

A guarantee is a strong promise that involves backing up your words with action. Companies that offer a “money-back guarantee” are so confident in their product that they'll return your money if you're not satisfied. When a school guarantees students' safety on a field trip, it is taking responsibility to help make sure students are safe.

The word can also be used as a noun: “This camera comes with a two-year guarantee” means the company promises to fix any problems for two years. People sometimes use guarantee to express absolute certainty: “I guarantee you'll love this book” means they're completely sure about it.

Be careful not to confuse guarantee with warranty, though they're similar. A warranty is specifically a written guarantee about a product, while guarantee has broader uses. You can guarantee anything from showing up on time to keeping a secret, not just the quality of merchandise.