OED 1.a. Rhetoric. (A figure of speech characterized by) the action of substituting for a word or phrase denoting an object, action, institution, etc., a word or phrase denoting a property or something associated with it; an instance of this.
1.b. In extended use: a thing used or regarded as a substitute for or symbol of something else. Also (esp. in Linguistics and Literary Theory): the process of semantic association involved in producing and understanding a metonymy.
Because the association involved in metonymy is typically by contiguity rather than similarity, metonymy is often contrasted with metaphor.
A figure of speech in which one word or phrase is substituted for another with which it is closely associated (such as "crown" for "royalty").
The White House asked the television networks for air
time on Monday night.
a figure of speech that replaces the name of one thing with the name of something else closely associated with it, e.g. "the bottle" for alcoholic drink, "the press" for journalism, skirt for woman, Mozart for Mozart's music, the Oval Office for the US presidency. A well‐known metonymic saying is the pen is mightier than the sword (i.e. writing is more powerful than warfare). A word used in such metonymic expressions is sometimes called a metonym [met‐ŏnim]. An important kind of metonymy is synecdoche, in which the name of a part is substituted for that of a whole (e.g. hand for worker), or vice versa.
"The pen is mightier than the sword."
"By the sweat of thy brow thou shalt eat thy bread." (Genesis 3:19)
wheels to mean 'car', brain to mean 'intelligent person', tongue for language
Roman Jakobson, "Two Aspects of Language . . . " (1956)
Psychology Today article on Jakobson's Metaphor & Metonymy