What defines progressive rock?

Related questions

1. Does yes still tour?
2. How much is the YES Network?
3. Is yes still touring?
4. How much is Bill Bruford worth?
5. Why did Bill Bruford leave Genesis?
6. Who carries the YES Network?
7. Who is playing bass for yes?
8. Is the band Yes still together?
9. Is Pink Floyd progressive rock?
10. What is the current lineup of Yes?

Tags

1. #throughout
2. #progressive
3. #genre
4. #symphonic
5. #1960s
6. #shortened
7. #united
8. #classical
9. #sometimes
10. #called
11. #states
12. #music
13. #developed
14. #broad
15. #kingdom