Metonymy: "The White House said today..." "The pen is mightier than the sword." "It's all about the Benjamins."
These are forms of metonymy, from the Greek meta- "transfer" and onoma "name." What you do is call something as something else with which it is closely associated. In fact, the examples above are so commonly cited, they might as well be metonymy for metonymy.
A special kind of metonymy is synecdoche, using a part for the whole, which we'll look at tomorrow.