Metonymy is a specific figure of speech, one in which a detail of a complex event or situation is summed up by one definite specific part of that event or situation. For example, a military man’s retirement is expressed as “He hung up his sword.” Every day on the news, you hear the entire Federal administration referred to as “The White House”—a clear metonymy reference. The figure of speech is reflective of our own mental process, of thinking of an event (the green grass of the picnic) or a situation (his face was red with anger) by remembering and expressing a detail that sums up the whole atmosphere or mood of an event. The way the brain works is by expanding a metonymy into a whole scene. When someone says “Do you remember your birthday party?” you remember first some strong moment—when you opened your big present, for example; then other details come to mind, and you say “Yes, I remember my birthday party.” That is how metonymy works. Poets, especially, use it for economy and rhythm: “She never would disgrace the ring she wore” (Byron).
It is the substitution of the name of an attribute or adjunct for that of the thing meant. In other words, it is a figure of speech in which one word or phrase is substituted for another which it is closely associated. For example, Washington for United States or crown for royalty.