Idiom

An idiom is a common word or phrase with a culturally understood meaning that differs from what its composite words' denotations would suggest; i.e. the words together have a meaning that is different from the dictionary definitions of the individual words.

By another definition, an idiom is a speech form or an expression of a given language that is peculiar to itself grammatically or cannot be understood from the individual meanings of its elements.

For example, an English speaker would understand the phrase "kick the bucket" to mean "to die" – and also to actually kick a bucket. Furthermore, they would understand when each meaning is being used in context.