We use synecdoche when we refer to something by using a term for one of its parts. Many common idioms in English include synecdoche.
- Lend me a hand, please. (The request is for physical help that involves the power of the whole body, not just a hand.)
- Would you like a mug? (‘Mug’ stands in for ‘mug of coffee/tea/cocoa/etc.; it is the container as well as what it contains.)
- Are you paying with plastic? (‘Plastic’ stands in for ‘credit card’, which is made up partly of plastic and which allows the user access to money.)
Less often, we use synecdoche when we use a whole to mean a part.
- South Africa won the soccer match. (The term ‘South Africa’ is used in place of ‘the South African national soccer team, Bafana Bafana’.)
- The world is a tough place. (here the speaker is referring to parts of the world)