Why is the ninth month called September?
late 14c., "part for whole or vice versa," from Medieval Latin synodoche, from Late Latin synecdoche, from Greek synekdokhe, literally "a receiving together or jointly," from synekdekhesthai "supply a thought or word, take with something else," from syn- "with" (see syn-) + ek "out" (see ex-) + dekhesthai "to receive," related to dokein "seem good" (see decent). Figure in which an attribute or adjunct is substituted for the thing meant ("head" for "cattle," etc.).
figure of speech in which a part represents the whole, as in the expression "hired hands" for workmen or, less commonly, the whole represents a part, as in the use of the word "society" to mean high society. Closely related to metonymy-the replacement of a word by one closely related to the original-synecdoche is an important poetic device for creating vivid imagery. An example is Samuel Taylor Coleridge's line in "The Rime of the Ancient Mariner," "The western wave was all aflame," in which "wave" substitutes for "sea." See also metonymy.