the best definition of an idiom is
An idiom is a set expression of two or more words that means something other than the literal meanings of its individual words. Adjective: idiomatic. Idioms are the idiosyncrasies of a language, says Christine Ammer. Often defying the rules of logic, they pose great difficulties for non-native speakers (The American Heritage Dictionary of Idioms, 2013). For an explanation of the idiom principle, see Examples and Observations below.