So, a metonym is a term that replaces the name of one thing with the name of something else closely associated with it. Some examples: Broadway for American theater, sweat for hard work, or the White House for the U.S. presidency. Metonymy establishes relationships of contiguity, and thus illuminates the "process of association by which metonymies are produced and understood" (ODLT).
Metaphor, on the other hand, establishes relationships of simliarity by using a word or expression to refer to one thing, idea, or action normally denoting another thing, idea, or action, so as to suggest some common quality shared by the two.
To put it more succinctly:
- metaphor: relationship of similarity
- metonym: relationship of contiguity
Oh, ok, one more comment on metonymy... We've been hearing a lot about Wall Street, posed in its opposition to Main Street, and I would say that both are used as metonyms. I believe that Wall Street is meant to stand in for large financial institutions and the stock market, and the term used because the actual Wall Street and its practices are something far removed from most Americans' daily experience, but I'm even more unclear of exactly what they were using Main Street to mean. An idealized, ficticious image of a "mainstream" America? Working-class citizens? The free market? American businesses in general?
And, ok, one more word about the (updated) Grad School Vocab Project: Can you believe that liminal (of, pertaining to, or situated at a threshold) was not in my spell check's dictionary? I've had to add it. What are they teaching the young people these days?