A figure of speech in which a part is made to represent the whole or vice versa, as in England lost by six wickets (meaning ‘ the English cricket team’).
The substitution of the name of an attribute or adjunct for that of the thing meant, for example suit for business executive, or the turf for horse racing.
There is a sense in which synecdoche is merely a special case of metonymy, meaning that metonymy is more general: the 'thing' substituted in metonymy doesn't have to be a component of the whole - it is just something closely associated (the 'part' can be argued to be closely associated to the 'whole'). So one could argue that anything classified as synecdoche can also technically be classified as metonymy.
If you wanted to distinguish metonymy from synecdoche, however, consider defining closely associated to exclude the part-whole relationship. This is probably closer to the way it's used in practice.
Here's a more extensive discussion of the difference:
Let’s take synecdoche first (which is pronounced as /sɪˈnɛkdəkɪ/ Help with IPA, by the way). You use this when you speak of a part of something but mean the whole thing. When Patrick O’Brian has Captain Jack Aubrey tell his first lieutenant to “let the hands go to dinner” he’s employing synecdoche, because he’s using a part (the hand) for the whole man. You can also reverse the whole and the part, so using a word for something when you only mean part of it. This often comes up in sport: a commentator might say that “The West Indies has lost to England” when he means that the West Indian team has lost to the English one. America is often used as synecdoche in this second sense, as the word refers to the whole continent but is frequently applied to a part of it, the USA.
Metonymy is similar, but uses something more generally or loosely associated with a concept to stand in for it. When Americans speak of the Oval Office, for example, they are really referring to the activity within it, the position or function of the President. It’s a linked term, and so a metonym. British writers refer similarly to the Crown, when they’re really discussing the powers, authority and responsibilities of the monarchy, which is symbolised by the crown. The difference between synecdoche and metonymy is that in metonymy the word you employ is linked to the concept you are really talking about, but isn’t actually a part of it. Another example is the turf for horse racing. But the distinction isn’t always obvious and often can’t be rigorously applied, and many people use metonymy to mean both.
- World Wide Words