Synecdoche is a figure of speech in which a part is used to represent the whole or vice versa. It can involve using a part to refer to the whole, a whole thing referring to a part, a general class name denoting a specific instance, or referring to materials something is made of. Some examples given include referring to people by a single characteristic like "the beard" for an older man, calling workers "hands", or saying "the law" to mean police officers.
The summary provides the key definition of synecdoche as a figure of speech where a part represents the whole or vice versa, and gives some brief illustrative examples to highlight this concept in 3 sentences.