It is a figure of speech in which a thing or concept is referred to by the name of something which is closely associated with that thing or concept. It is a figure of language where instead of using the actual name of something, we use the name of something else which is closely related to it or which resembles it. A metonymy is simply a substitution where a word or phrase is used in place of another word or phrase.
√ “The pen is mightier than the sword.” “Pen” stands for “the written word and “sword” substitutes violence or military force.
√ Crown – in place of a royal person /government/authority.
√ The White House or The Oval Office – used in place of the American President or White House staff.
√ Suits – in place of business people.
√ Heart – to refer to love or emotion.
√ Washington – to refer to the US government.
√ Ears – for giving attention, listening.
√ Hand – for help.
√ Tongue – used in place of language.
√ Hollywood – to refer to the film industry.
√ New blood – used in place of new people, fresh ideas.
√ The chair has called of the meeting. Chair=person.
√ The bench usually refers to the judges.
√ Dish to refer to an entire plate of food.
√ The big house – to refer to prison.
√ Silicon Valley – to refer to the technology industry.
Metonymy and synecdoche resemble one another because they both use a word or phrase to represent something else. They are both considered as forms of metaphor. Either metaphor, or metonymy or synecdoche involves the substitution of one word for another that requires conceptual link. Synecdoche can also be a form of personification when the non-human thing substitutes a human element.
The main difference is that synecdoche uses the part of the thing it represents or the whole thing to mean its part. On the other hand, metonymy doesn’t use the part for the whole or the whole for the part, but rather uses a term that is related to the thing it means.