What to Know
Synecdoche refers to a figure of speech in which the word for a part of something is used to refer to the thing itself (as hired hand for “worker”), or less commonly, the word for a thing itself is used to refer to part of that thing (as when society denotes "high society"). In metonymy, a word that is associated with something is used to refer to that thing (as when crown is used to mean "king" or "queen").
The terms metonymy and synecdoche refer to two similar figures of speech often used as literary devices. (They're easy to confuse, so feel free to read this as many times as you need.)
What is Synecdoche
Synecdoche refers to a figure of speech in which the word for a part of something is used to refer to the thing itself, or less commonly, when the word for a thing itself is used to refer to part of that thing. The first kind of synecdoche is what we hear when someone uses wheels to refer to a car (“she showed off her new wheels”) or threads to refer to clothing (“a new set of threads”); the second kind is what’s going on when a phrase like “introduced to society” is used to talk about an introduction specifically to high society.
A classic example of synecdoche is the use of the term hands to mean “workers” (as in “all hands on deck”), or the noun sails to mean “ships.” Synecdoche is also sometimes used in the names of sports teams, e.g., the White Sox, the Blue Jackets.
What is Metonymy
Metonymy refers to a figure of speech in which the word for one thing is used to refer to something related to that thing, such as crown for “king” or “queen,” or White House or Oval Office for “President.” The phrase “a bunch of suits” for a group of businesspeople is an example of metonymy; it uses the common wardrobe of businesspeople as shorthand for the people themselves.
It’s metonymy when you use a person’s name to refer to the works by that person, as when you say “We’re reading Austen this semester” when you really mean “We’re reading works by Austen this semester.” And it’s metonymy when you use a city’s name to refer to its team, as when you say “Houston was ahead by six points.”
Some examples of metonymy are so common as to have become a regular part of the lexicon. The use of press to mean “journalists” dates to the 17th century and occurs in the First Amendment (“Congress shall make no law … abridging the freedom of speech, or of the press…”). It identifies journalists with the name of the device used for printing newspapers.
Similarly, a sportswriter might say that a team’s bats went into a slump, when what the writer really means is that the hitters in the lineup went into the slump. Sometimes metonymy is used to make a name catchier than the item it replaces, like when surf and turf, a phrase employing two rhyming terms that allude to the sea and land, is used for a dish combining seafood and beef.
Like many terms used in rhetoric, both synecdoche and metonymy derive from Greek. The syn- in synecdoche means "with, along with" (much like as in synonym) and ekdochē means "sense, interpretation." Metonymy meanwhile, combines the Greek meta (“among, with, after,” the same root found in metaphor) with ōnymon, meaning “name” or “word.”
Now that you've got these two figured out, check out this list of other common rhetorical devices.