An idiom is a phrase whose meaning is different from the meanings of each of its words considered separately. These phrases have a fixed form – they usually cannot be changed – and they are often informal, but they can also be slang, rude slang, or even slightly formal. Many idioms are used in spoken English, but they also appear in newspapers and magazines, in books, and in academic writing.

Not all fixed phrases are idioms. For example, close your eyes is a common fixed phrase, but it is not an idiom, because each word in it is used in its standard meaning. The phrase keep your shirt on is an idiom, however, because the phrase does not mean “do not take off your shirt” – it means “stay calm.”