a stuffy college professor word that describes when you use part of an object to refer to the whole object. If you call you gun a "nine," or refer to a guy who is mean as a "dick," thats synecodche. Like most college professor words, it is originally greek, transferred into Latin and then imported into english.
"Man I can't go out tonight. I got no wheels"
(since he says "wheels" but means "car," this is synecdoche.)