From Wiki:
the style was an emergence of psychedelic bands who abandoned standard pop traditions in favour of instrumentation and compositional techniques more frequently associated with jazz, folk, or classical music. Additional elements contributed to its "progressive" label: lyrics were more poetic, technology was harnessed for new sounds, music approached the condition of "art", and the studio, rather than the stage, became the focus of musical activity, which often involved creating music for listening rather than dancing.
So Yeah, Boston and Styx dabbled in the Prog Rock. They were also considered Arena Rock.
Yeah, and ask ten people what Progressive Rock is and you'll get, well at least 2-3 different answers. I looked up the difference between art rock and prog rock once. They described the characteristics of each, where there was probably 80% overlap, and the differences, which were few. And when they listed bands as examples of each, there was again about 80-90% overlap. The one person I've seen described as Art Rock whom I wouldn't consider progressive rock is Tori Amos (although anyone, including me, is free to post something from her).
I've been surprised in recent years to find that a lot of people consider bands like the Mahavishnu Orchestra and Weather Report to be prog rock. Since John McLaughlin, Jan Hammer, Jerry Goodman, Joe Zawinul, Stanley Clarke, Billy Cobham, all grew up in jazz, I'd always considered them progressive jazz.
But the tent is big, and nobody has a monopoly on what constitutes what, because there isn't a hard-and-fast definition. I've enjoyed everything posted, and learned of a few new bands, whom I'll visit on YT in the coming days. Y'all have beat me to some bands, but I'll post something more from Camel, King Crimson, and maybe ELP or Yes. Keep it coming!