Pop is simply short for popular. What pop music has always been
is just what was popular at the time. At one point pop music was
just theater productions. Many years later was Little Richard,
Elvis, and then the British revolution including the Beatles and
Buddy Holly came along. The 70's and 80's were primarily rock, then
the late 80's and 90's brought pop music closer to what it is today
with Madonna and Michael Jackson, then Britney Spears, the
Backstreet Boys, and N Sync brought us into the new decade.