What does the term 'idiom' specifically refer to?

An idiom specifically refers to a figure of speech where the combined meaning of the words differs significantly from the meanings of the individual words when taken separately. This unique characteristic makes idioms a fascinating aspect of language, as they often reflect cultural nuances and cannot be understood merely by analyzing the individual words. For example, the phrase "kick the bucket" does not literally mean to kick a bucket; instead, it is an idiom that means to die. This distinction highlights why the correct choice clearly encapsulates the essence of what an idiom is in language.