A synecdoche is a rhetorical and literary device in which a part of a system or group represents the whole.  Also, a synecdoche can use a whole to represent a part.  Synecdoche and metonymy differ in that the synecdoche refers to the whole of a thing by the name of any one of its collective parts while metonymy refers to when a word which describes another thing is closely or directly linked to that particular thing, but it does not have to be a part of it.

Literary Example:

He asked for her hand in marriage

Referring to people or the quantization of people by body part: e.g. head count.

Visual Example: