Word of the day: synecdoche
Synecdoche is a figure of speech in which a part is used for the whole (as hand for sailor), the whole for a part (as the law for police officer), the specific for the general (as cutthroat for assassin), the general for the specific (as thief for pickpocket), or the material for the thing made from it (as steel for sword).
You can find a fuller explanation here.
The word also occurs in the title of a current film.