I have been a prog. fan since the mid seventies, and I have seen this word progressive thrown around alot, progressive jazz, synth, prog. rock. To me progressive rock is music that lends itself to no rules, unlike many of the bands of the 70's and 80's which followed musical formulas, progressive bands make it up as they went along. So it is really the integrity of the music written and played that makes progressive rock what it is.
Be it fusion, classical, rock, jazz....
