A pop quiz is a term used particularly in US high schools, referring to a teacher giving a quiz to the class without prior warning. This is usually done in order to regain a classes focus on a certain subject that they may otherwise not have been studying hard enough.

The movie Speed (1994) also used this term as a way to verbally attack someone by asking them what they would do in a crisis situation.

"Pop Quiz" is also an offshoot of "pub quiz" where all the questions were related to pop music themes.

(from WIkIPEDIA)