Definition - A figure of speech where a part of something is used to refer to the whole (and vice versa).
Note: As opposed to metonymy where a thing that is associated with something but is not part of it is used to refer to it, e.g. calling the ocean the deep.

Example -

(1) Saying hired hands instead of workers (part for whole) and
(2) Saying the law instead of the police (whole for part)

Etymology -
The term derives from the Greek synekdokhe, a receiving together (from syn, with + ek, out + dekhesthai, to receive).

Oxford English Dictionary -
Its first citation is from 1388:
"Bi a figure clepid synodoches [v.r. synadochie], whanne a part is set for al, either al is set for oo part."
(Wyclif's Bible, Prol. xii. (1850) 47)

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