A word that denotes one thing but refers to a related thing. "Plastic is a metonym for credit card."Websters elaborates on that:
A figure of speech consisting of the use of the name of one thing for that of another of which it is an attribute or with which it is associated, as "crown" in "lands belonging to the crown."These are metonyms because:
- Credit cards are made of plastic.
- The king (or queen) wears a crown.
- The White House called today. (Someone who lives or works in the White House called.)
- I'm thirsty for some suds. (Beer has suds; I want a beer.)
- You can rent some fancy wheels. (Cars have wheels; you can rent fancy cars.)
- It is a synecdoche if it is an actual component of the thing referenced (like "plastic,", "suds," and "wheels," above).
- It is a metonym if it is closely associated with the thing referenced but isn't actually a part of it (like "crown" (I think--although one could maybe argue it's a synecdoche also) and "White House").