Modern slang for jam, it's used to describe a really good, modern song. For example, "The new Justin Bieber song is a bop."

Historical perspective: A great example of language fluidity, the musical term "bop" was originally referred to as "bebop" to describe a style of jazz developed in the early to mid-1940s in the United States, which features songs characterized by a fast tempo, complex chord progressions with rapid chord changes. Now it's Justin Bieber.

NetLingo Classification: Online Jargon