Idioms - part 1.

An idiom is typically defined as a phrase relevant to a certain culture which can't be understood to an outsider by its component parts. Sound like a mouthful? Good. For example, the phrases sour grapes, white elephant, elephant in the room and monkey's uncle are all idiomatic phrases. (I.e. only an English person can understand them - they would have to be explained in full to non-English people.)