To err means to make a mistake or be wrong about something. When you err in your math homework, you've calculated incorrectly. When a referee errs during a game, they've made the wrong call. The word suggests an honest mistake rather than intentional wrongdoing.

You'll often hear the phrase to err is human, which reminds us that everyone makes mistakes sometimes. A judge might err in their ruling, a scientist might err in their calculations, or you might err when you think your friend's birthday is next week when it's actually today.

The word error comes from the same root and means the mistake itself. If you err on a spelling test, each misspelled word is an error. People sometimes say they'll err on the side of caution, meaning if they're unsure, they'll choose the safer option. For example, if you're not sure whether to bring an umbrella, you might err on the side of caution and bring it anyway.

Notice that erring implies you meant well but got something wrong, unlike cheating or lying, which involve doing wrong on purpose.