This is an interesting one. In many ways, a metonymy is the opposite of a metaphor but they both do the same thing. A metaphor compares a thing to something unrelated by way of implication or suggestion (a brilliant shining diamond bore down in them from the sky) and a simile is a direct comparison (the sun was like a brilliant jewel that morning).

In contrast, metonymy is the direct replacement of one word or term or another for something associated with it rather than implying or drawing a comparison. Also unlike the other two, there is already a close association, so it is less figurative and more a literal thing.

As the most common example from here in the UK, we call vacuum cleaners “hoovers” after a very successful brand in the 1970s. Even today, despite that Dyson and Henry are battling it out for the top of the pile and few people have hoovers, we still refer to these devices colloquially by the previous successful brand name. Why do we do this? Most likely because “hoover” rolls off the tongue easier than “vacuum cleaner” and probably partly because Dyson also manufacture other goods.

Here are a few other typical examples of metonymy:

• Business people are “suits”
• The entire American film industry is referred to as “Hollywood”, even if the studio is not based anywhere near California
• Similiarly “Wall Street” refers to the US Stock Exchange and
• “The City” is used to refer to the financial sector of London (even though The City of London is a district of the capital, it is where most of these financial services are located)
• The pen is mightier than the sword –  both words are metonymy here. “The pen” means the written word and not an actual physical pen. The sword, again, is not a literal sword but the symbol of military power
• Referring to food as “dish” – instead of a single course of a meal. A dish when taken literally is actually a piece of crockery
• “Lend me your ears” – not literally because that would include disfigurement. What Marc Antony meant in Shakespeare’s Antony & Cleopatra was for people to listen to him
• Referring to somebody as an animal because of their behaviour: pig (male) or cow (female) for being unpleasant. Monkey is used for children who are being cheeky or playfully devious. Other animals we call people are “bitch”, “slug”, “snake”, “cougar”, “fox”, “weasel”

Functions of Metonymy?

Like other forms of stylistic writing, it’s to add depth and colour to the writing. Few writers write literally all the time and if we did, that would be incredibly boring for us to write and for you to read with no more stylistic passion than a shopping list or a set of electronics programming instructions. Those examples above – particularly the Shakespeare example and the “pen is mightier than the sword” work and stick in the mind because of the metonymy.

A second way in which we use metonymy is for convenience. Repeated use of a lengthy phrase can get tedious but using a short and punchy byword can add to flow of the text.

• I couldn’t be bothered to cook so I had a KFC: You had food from KFC, you did not eat the building and the contents of the building including the employees so you refer to the company as a byword for the food you ate. It’s simply easier to say “I had KFC / McDonalds / Burger King”
• Referring to Europe and North America as “The West”: The Earth is a sphere so there is no “west”. What we understand of the political west should refer to the Anglo-European sphere of influence with a Cultural Christian heritage rather than an Islamic or Buddhist one but “The West” is easier to remember and repeat than “North America and Europe and the area of the world where Christianity is largely predominant”
• Downing Street & Westminster / The White House: The example is the same but this is when we refer to buildings as the essence of our government; in the first case referring to the British government as “Downing Street” and Parliament as “Westminster”, in the second it’s referring to the President and his staff as the building itself.
• The Bard: Taken literally, we would think that Shakespeare was the only bard, but he wasn’t. Yet we know who we mean when we use this title.
• My new wheels: This means a new car or other motorised vehicle. We use “wheels” to refer to the entire construction and not just the physical wheels