Expiration is the end of the time period when something is valid, safe, or usable. When milk reaches its expiration date, it may no longer be safe to drink. When your library card expires, you need to renew it before you can check out more books.

A coupon expires when its deadline passes. A parking meter expires when your paid time ends. Medicine has expiration dates because the chemicals inside can break down and become less effective or even dangerous.

Expiration dates help by telling us when food, medicine, or other products are no longer guaranteed to work properly. Some expiration dates are strict rules (like for medicine), while others are suggestions (many canned foods stay safe well past their printed dates). People often check expiration dates before buying, and many keep track of when important documents like passports or memberships expire.

The related verb is expire: “My gym membership expires next month.” When something has expired, it's past its expiration date and may no longer be valid.