An alibi is proof that you were somewhere else when something happened, making it impossible for you to have done it.

If someone's bike gets stolen from the playground during lunch, but you were in the library the whole time with the librarian helping you find books, that's your alibi. You have proof you were elsewhere when the theft occurred. In detective stories and courtroom dramas, establishing an alibi often becomes crucial: if a suspect can prove they were across town eating dinner when a crime happened, they couldn't possibly be guilty.

People sometimes use alibi more casually to mean any excuse, saying things like “What's your alibi for not finishing your homework?” But this isn't quite right. A true alibi is specific evidence that places you in a different location at a particular time, making it physically impossible for you to have been involved in whatever happened. An alibi either holds up under scrutiny or it doesn't. If your “I was at the library” alibi falls apart because the librarian doesn't remember seeing you, then you don't really have one.