Synecdoche is a figure of speech in which a part stands for the whole or the whole for a part.
Synecdoche, from the Greek for 'simultaneous understanding', has been catalogued in rhetorical handbooks since classical antiquity. It works in two directions: 'sails' for ships, 'wheels' for cars, 'boots on the ground' for soldiers — each substituting a part for the whole. Or the other way: 'America' for a single American, 'the law' for a police officer. The trick is so woven into ordinary language that we rarely notice it, but writers use it deliberately to compress images and shift attention. Walt Whitman's 'I see the boatmen and clamdiggers arise early and stop late' uses occupations to evoke entire lives. The figure is closely related to metonymy and the two are often confused, but synecdoche specifically requires a part-whole or whole-part relationship, not mere association.