Synecdoche
Overview
Synecdoche is a figure of speech
Figure of speech
A figure of speech is the use of a word or words diverging from its usual meaning. It can also be a special repetition, arrangement or omission of words with literal meaning, or a phrase with a specialized meaning not based on the literal meaning of the words in it, as in idiom, metaphor, simile,...

 in which a term is used in one of the following ways:
  • Part of something is used to refer to the whole thing (pars pro toto
    Pars pro toto
    Pars pro toto is Latin for "a part for the whole" where the name of a portion of an object or concept represents the entire object or context....

    ), or
  • A thing (a "whole") is used to refer to part of it (totum pro parte
    Totum pro parte
    Totum pro parte is Latin for "the whole for a part"; it refers to a kind of synecdoche. When used in a context of language it means that something is named after something of which it is only a part...

    ), or
  • A specific class of thing is used to refer to a larger, more general class, or
  • A general class of thing is used to refer to a smaller, more specific class, or
  • A material is used to refer to an object composed of that material, or
  • A container is used to refer to its contents.

Synecdoche is closely related to metonymy
Metonymy
Metonymy is a figure of speech used in rhetoric in which a thing or concept is not called by its own name, but by the name of something intimately associated with that thing or concept...

 (the figure of speech in which a term denoting one thing is used to refer to a related thing); indeed, synecdoche is sometimes considered a subclass of metonymy.
 
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