The Wikipedia definition actually pretty much nails it: "Classic rock is a radio format which developed from the album-oriented rock (AOR) format in the early 1980s. In the United States, the classic rock format features music ranging generally from the late 1960s to the late 1980s, primarily focusing on commercially successful hard rock popularized in the 1970s."
When a genre tag originates as a radio format, it's always going to be a bit vague.
|