Synecdoche is a kind of metonymy in which part of something is used for the whole.
Some common examples of synecdoche are "hands" to refer to workers, "head" to refer to cattle, "threads" to refer to clothing and "mouths to feed" to refer to hungry people.
Synecdoche, as well as other forms of metonymy, is one of the most common ways to characterize a fictional character. Frequently, someone will be consistently described by a single body part or feature, such as the eyes, which comes to represent their person.
Also, sonnets and other forms of erotic love poetry frequently use synecdoches to characterize the beloved in terms of individual body parts rather than a whole, coherent self.