Why is the ninth month called September?
1560s, from French métonymie (16c.) and directly from Late Latin metonymia, from Greek metonymia, literally "a change of name," related to metonomazein "to call by a new name; to take a new name," from meta- "change" (see meta-) + onyma, dialectal form of onoma "name" (see name (n.)). Figure in which the name of one thing is used in place of another that is suggested by or associated with it (e.g. the Kremlin for "the Russian government"). Related: Metonymic; metonymical.
metonymy me·ton·y·my (mə-tŏn'ə-mē)
In schizophrenia, a language disturbance in which an inappropriate but related word is used in place of the correct one.
(from Greek metonymia, "change of name," or "misnomer"), figure of speech in which the name of an object or concept is replaced with a word closely related to or suggested by the original, as "crown" to mean "king" ("The power of the crown was mortally weakened") or an author for his works ("I'm studying Shakespeare"). A familiar Shakespearean example is Mark Antony's speech in Julius Caesar in which he asks of his audience: "Lend me your ears."