Definition - Referring to something by naming
(1) one of its parts or
(2) something that is associated with it.

Example -
Calling a spokesman for the president of the United States a White House spokesman.

Etymology -
The word derives from the Greek metonymia, a change of name (from the Greek meta, change + onoma, name).

Oxford English Dictionary -
Its first citation is from 1547:
"Men seyth that they admyt metonomian, and say under the forme of breade is the trew bodye of Christ."
(Hooper Answ. Winchester's Bk. D1b)

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