Almost impossible to come up with a definition of Prog that satisfies everybody. That said, I think that there are two main elements that
must be present in order to get close enough for just about anyone:
1 - Frequent use of time signatures other than 4/4 and 3/4. In other words, 'regular folks' (who might wear tennis shoes or an occasional python boot) can't dance to it
2 - Lyrics that go anywhere other than the usual variations on wanting/having/missing out on sex. Bonus points for shoehorning in references to one or another sort of imminent Apocalypse
As a matter of personal opinion, ELO fails utterly due to point #1. Despite a few nifty non-standard touches here and there, they seemed firmly rooted in 4/4 time
IMHO the bulk of Frank Zappa's work hits both nails right on the head. Yeah, I'm well aware of songs like
Dirty Love and the like, but if you stack those up against his total output they're actually a fairly small fraction. OTOH Frank tended to write lyrics that were firmly grounded in observed reality - not fantasy or speculation. In the end, he was sooooo unique that he wouldn't neatly fit into any category besides "Zappa"
Now, I could go on like this for another 20 pages (which probably means I fit the Prog demographic pretty well

) so lemme just cut to the chase. My Top 5:
Gentle Giant
Yes
Rush
Emerson, Lake, and Palmer
The Aristocrats (who needs lyrics when you're that damned good)