Metonymy

Metonymy - a figure of speech that consists in using the name of one thing for that of something else with which it is associated.

Examples:
"They counted heads" as opposed to "they counted the people".
"The pen is mightier than the sword." - "The pen is an attribute of thoughts that are written with a pen; the sword is an attribute of military action."*



Sources:
*http://rhetoric.byu.edu/figures/m/metonymy.htm