What is synecdeche?
It is a form of metonymy, in which a thing or concept is referred to by the name of one of its parts.
* For example:
"All hands on deck!" (refers to the entire crew of a ship, not just their hands)
"The pen is mightier than the sword." (refers to the power of writing and ideas, not just the literal pen)
"Give us this day our daily bread." (refers to all of our daily needs, not just bread)