The figure of speech that uses a part to represent the whole, like “wheels” meaning a car, is synecdoche.
Synecdoche works through substitution between a part and a whole. Saying “wheels” for an automobile is an example of pars pro toto, Latin for “part for the whole.” The reverse also occurs: totum pro parte uses a whole to refer to one portion, as when “the world” means a particular country or region.
The word comes from the Ancient Greek synekdokhē, meaning “simultaneous understanding.” Synecdoche is generally treated as a type of metonymy, but the distinction is useful: in synecdoche, the substituted term is part of the thing or contains it. In metonymy, the substitute is merely associated with it; “the crown” for a monarch is a classic example.
Everyday English is full of synecdoches. “Boots on the ground” can mean soldiers, “suits” can mean businesspeople, and “I need a hand” can mean that someone needs an entire person’s help. The device is also common in poetry, advertising, politics, and sports reporting.