What is the difference between Metaphor and Metonymy?

Metonymy is a figure of speech. It is used in rhetoric where something is not referred to by its proper name, but rather it's associated word. A metaphor is an expression. The expression displays a similarity between two things in some respects. In metonymy, the relationship with the word is based on contiguity. With a metaphor, change is based on similarity, and if a metaphor can be used to define the conversion of the relation between sets of things.

Metonymy is used to define a word. It uses a single element for the naming of a complex entity. Another contrast between metaphor and metonymy is that metaphor acts by combining ideas. Both metaphor and metonymy are utilized to express ideas that are vastly different from the original meaning.