Synecdoche
Synecdoche, wherein a specific part of something is used to refer to the whole, usually is understood as a specific kind of metonymy. Sometimes, however, people make an absolute distinction between a metonymy and a synecdoche, treating metonymy as different from, rather than inclusive of, synecdoche. There is a similar problem with the use of simile and metaphor.
When the distinction is made, it is the following: when "A" is used to refer to "B", it is a synecdoche if A is a component of B and a metonym if A is commonly associated with B, but not part of its whole.
Thus, "The White House said" would be a metonymy for the president and his staff, because the White House (A) is not part of the president nor of his staff (B), but is closely associated with them. On the other hand, "20,000 hungry mouths to feed" is a synecdoche because mouths (A) are a part of the people (B) referred to.
One example of a simple sentence that displays synecdoche, metaphor, and metonymy is: "Fifty keels ploughed the deep", where "keels" is the synecdoche, as it names the whole (the ship) after a particular part (of the ship); "ploughed" is the metaphor, as it substitutes the concept of ploughing a field for moving through the ocean; and "the deep" is the metonym, as "depth" is an attribute associated with the ocean.
Read more about this topic: Metonymy