Noun
an expression whose meanings cannot be inferred from the meanings of the words that make it up
Source: WordNetThe greatness of Mac Rebennack, alias, Dr. John, also known as John Crieux, rests on his command of the musical use of idiomatic expression. Not a technically well-endowed singer, nor a great songwriter, he leaves his mark through the discipline and control he exerts over all that he touches. Jon Landau