How we change what others think, feel, believe and do
Metonymy is the use of one item's name to represent another item. In particular the representing item usually has a close association with the represented item.
Henry invaded France. (Henry represents Britain)
The pen is mightier than the sword. (Pen represents words. Sword represents military power).
I'll just Google it. (using Google instead of search).
Metonymy is quite close to metaphor, but with a more specific use. Metonymy uses names, whilst metaphor can indicate any representation.
Metonymy substitutes the contained for
Metonymy can be used in a number of associations, for example:
A common use of metonymy is in synecdoche, where a part represents the whole, for example the monarch being described as 'the crown'.
Metonymy comes from the Greek 'metonymia' meaning 'a change of name'.