Metonymy

metonymy: (from the Greek: "changed label" or "substitute name") a figure of speech in which the name of one object is substituted for that of another closely associated with it

"BIG BROTHER IS WATCHING YOU,"
                                                              -1984
                                                       by George Orwell

Big Brother is not in actuality watching the citizens of Oceania, but the Party is. Big Brother is the "leader" of the Party, and as such replaces the Party in the posters all over the city. The use of Big Brother rater than the Party creates a more personal feel, the people may feel both intimidated and protected by their "big brother" a figure usually looked up to in the atomic family. However, in this case Big Brother is the totalitarian dictator of the world, but is much more intimate than the Party, and is therefore used to replace the Party. Orwell uses the metonymy here to communicate the awfulness of the situation, one man has taken over the entire world and rules it harshly, scrutinizing every movement so thought even thoughts do not go unpunished. Orwell wants the reader to be horrified and seek to change their ways, and the ways of the world to avoid such a predicament in the future.