Synecdoche (the E is pronounced like in recipe) is a literary device in which a part of something is used to refer to the whole (or vice versa). It’s mainly considered a poetic device, and most examples that people provide are from poems. In this passage from Samuel Taylor Coleridge’s “The Rime of the Ancient Mariner,” for example, the word wave stands in for the sea:
The western wave was all a-flame.
The day was well was nigh done!
Almost upon the western wave
Rested the broad bright Sun
We use the word wave similarly when we use the phrase on the wave. Similarly, we can say a bird is on the wing when it’s flying. These are the two examples I was given when I was introduced to the term, back in, I believe, my final year of primary school. I was intrigued by this strange-sounding and -looking word, and wanted to think of other examples, though nothing really came to mind. I soon came to think of it as a purely literary device that was interesting, but not hugely relevant to my life. But now wiser and with a wider vocabulary, I can see that synecdoche is actually fairly common.
Politics is probably the are wherein synecdoche is most evident. Think about how we might say something like:
Russia has been implicated in influencing the election.
Obviously we don’t mean the whole country, but the Russian government. In this case we’re using the less common version of synecdoche: the whole referring to the part. Similar to synecdoche is metonymy, when something closely related to something, but not part of it, is used to represent it. For example:
The White House has denied the allegations.
Here the White House represents the American government, but this is a case of metonymy, and not synecdoche, as the White House is associated with the government, but not part of it. Similarly, we can refer to the British Government as No. 10, and the EU as Brussels.
Synecdoche can be used in more common speech though. Consider the word glasses: only part of a pair of spectacles is made of glass, so that counts as synecdoche. Likewise in military parlance when tanks are referred to as armour. Even in more idiomatic language we can find synecdoche. Consider how in American English, driving a car (or wheels) with manual transmission is referred to as driving stick. Hippies were commonly referred to as long hairs in the past, and if they were protesting, they ran the risk of being put behind bars. Which is also synecdoche when you think about it, as being behind bars is only part of the experience of being in prison (if a pretty major part). In fact, you could say that the term in prison is an example of synecdoche. Being in prison does not just refer to being inside a prison. It refers to the whole experience of having one’s liberty taken away and being put in prison. Contrast how we say that someone working in or visiting a prison is in a/the prison:
My cousin is in prison for stealing a car.
My brother works in a prison as a guard.
We do similar things with other phrases. To be in school or university specifically means to be learning, whereas a teacher works in a school. Equally, being in hospital means not just physically being inside a hospital, but also being sick/injured and being treated accordingly. Usually synecdoche refers to parts of concrete objects being used to refer to the whole, whereas in these cases we’re using a part of an experience to represent the whole. It might not meet everyone’s definition of synecdoche, but it’s close enough for me.
So while synecdoche might sound like a fancy, pretentious poetic device, it’s actually quite a common item of speech.