Synecdoche

Here's a hard one for you today! Synecdoche

\sə-ˈnek-də-kē\

Definition:
A figure of speech by which a part is put for the whole (as fifty sail for fifty ships), the whole for a part (as society for high society), the species for the genus (as cutthroat for assassin), the genus for the species (as a creature for a man), or the name of the material for the thing made (as boards for stage)

Etymology:
Latin, from Greek synekdochē, from syn- + ekdochē sense, interpretation, from ekdechesthai to receive, understand, from ex from + dechesthai to receive; akin to Greek dokein to seem good
 
Now, this is more common that you may think. From the definition, it is still a little difficult to grasp the true meaning, and the wildly abundant usage of this linguistic term. For a plethora of examples of usage, I turned to Wikipedia.
 
A part referring to the whole

• Prominently used in slang and vulgar speech, where the entire person is referred to by their genitalia.
• Referring to people according to a single characteristic: "the gray beard" representing an older man or "the long hair" representing a hippie. This leads to bahuvrihi compounds.
• Describing a complete vehicle as "wheels"
• Calling workers "hands", e.g. Many hands make light work; All hands on deck!
• Before and during the Cold War, the Soviet Union was commonly referred to by its largest and most well-known member, Russia.
• Use of the names England (only one of the four constituent nations) or Great Britain (the geographical name of the main island) to mean the entire United Kingdom.
• Use of Holland, a region of the Netherlands, to refer to the entire country

The name for a whole when used to describe one part of it

• In the United States, terms like "United States", "(the Commonwealth of) Virginia" or "(the) People (of the State of California)" are used in court trials when the plaintiff, prosecutor or defendant is a government entity, such as in Loving v. Virginia.

A general class name used to denote a specific member of that or an associated class

• "the good book," or "The Book" for the Bible
• "truck" for any four-wheel drive vehicle (as well as long-haul trailers, etc.)
• "He's good people." [Here, the word "people" is used to denote a specific instance of people, i.e. a person. So the sentence would be interpreted as "He's a good person.")

A specific class name used to refer to a general set of associated things

• "John Hancock" for the signature of any person
• a genericized trademark, for example "Coke" for any variety of cola or "Band-Aid" for any variety of adhesive bandage
• "bug" for any kind of insect or arachnid, even if it is not a true bug

The material that a thing is (actually, historically, or supposedly) made of referring to that thing

• "glasses" for spectacles
• "steel" for a sword
• "tin" for a container made with tin plating
• "willow" for a cricket bat or "pigskin" for an American or Canadian football
• "wood" for a type of club used in the sport of golf
• "irons" for shackles placed around a prisoner's wrists or ankles to restrict their movement
• "plastic" for a credit card (asking a merchant) Do you take plastic?
• "lead" for bullets (e.g. They pumped him full of lead.)
• "silver" for flatware or other dishes that were once made of silver metal
• "rubber" for a condom
• "threads" for clothing Yo, check out my new threads!

A container is used to refer to its contents

• "barrel" for a barrel of oil
• "keg" for a keg of beer

See? You probably use this figure of speech several times a day. Now you know what it's called!