Progressive rock (also referred to as prog rock or /prog/) is a subgenre of rock music[1] that developed in the late 1960s and early 1970s as part of a "mostly British attempt to elevate rock music to new levels of artistic credibility."[2]
[1] Listening to the future: the time of progressive rock, 1968-1978, pp. 71-75
[2] "Prog-Rock/Art Rock". AllMusic. AllMusic. 2007. http://allmusic.com/cg/amg.dll?p=amg&sql=77:374. Retrieved 2007-12-04.