A figure of speech in which a word referring to one attribute of something is used to signify the whole of the thing. For example “the crown” is used to signify “the monarchy”, and in the expression “he's taken to the bottle” one means “he's taken to drinking”. The difference between metonymy and synechdoche (q.v.) is fine: metonymy is relatively abstract and ideational whereas synecdoche is more material and physical (“sails” for “boats”).
Editors, Litencyc. "Metonymy". The Literary Encyclopedia. First published 01 November 2001
[https://www.litencyc.com/php/stopics.php?rec=true&UID=722, accessed 26 May 2019.]