To replace means to put something new in the place of something else. When a light bulb burns out, you replace it with a fresh one. When your shoes wear out, you replace them with a new pair. When a team's star player gets injured, the coach sends in another player to replace her on the field.

Replacing can be temporary or permanent. A substitute teacher replaces the regular teacher for a day or two, but eventually the regular teacher returns. But when you replace the batteries in your flashlight, those old batteries are gone for good.

The word suggests that whatever is being replaced either isn't working anymore, isn't good enough, or simply needs to be updated. Companies replace outdated computers with newer models. Doctors replace damaged joints with artificial ones. A friendship doesn't get replaced the same way a broken toy does: people and relationships are irreplaceable, meaning nothing and no one can truly take their place.

You can also use replace to mean putting something back where it belongs, like when a librarian asks you to replace books on the shelf after you're done reading them.