An idiom is a common word or phrase with a culturally understood meaning that differs from what its composite words’ denotations would suggest. 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. An idiom is not to be confused with other figures of speech such as a metaphor, proverbs, which are simple sayings that express a truth based on common sense or practical experience.
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.