In rhetoric, metonymy is the substitution of one term for another that is generally associated with it. Unlike metaphor, in which the substitution is based on similarity, the substitution in metonymy is based on an understood association between the two terms.
Examples of Metonymy
Referring to "suits" instead of "businessmen." Although the businessman is not his suit, he is frequently associated with it, and thus the substitution is understood.
Referring to "the home office" instead of "the management." Although the members of the management are not the home office, they are associated with the office; again, the substitution is understood.