metonym, noun.

1. A word that denotes one thing but refers to a related thing.

- Example: "Washington is a metonym for the United States government."
- Example: "Plastic is a metonym for credit card."

Before you protest, notice that the above is irony in action.

irony, noun.

1. Incongruity between what might be expected and what actually occurs;
2. A trope that involves incongruity between what is expected and what occurs;

trope, noun.

1. The use of a word or expression in a sense different from that which properly belongs to that word or expression.
2. The use of a word or expression as changed from the original signification to another, for the sake of giving life or emphasis to an idea.
3. A figure of speech.