Metonimy. A word is used for something 'related to' that which it usually refers to; for example, 'eye', 'skirt', 'breathe' in Keep your eyes on the ball [gaze] He’s always chasing skirts [girls] It won’t happen while I still breathe [live] Body parts are favourite sources of metonimy, and many such expressions have been incorporated into the language, with words like 'hand', 'heart', 'head' as in 'have a hand', 'bare one’s heart', 'keep your head'. The nature of the relationship is very varied, but is often something like cause, or source, or instrument. [...] These are generally thought of as lexical, or lexicosemantic, processes, with synecdoche being based on meronymy, and metaphor and metonymy on kinds of synonymic relationship. Alternatively, we can interpret them grammatically, in terms of the relational processes [...].
- Halliday (1985), Pag. 319-320