To me, Progressive Rock has always meant the following things. I don't know if this is accurate:

- Rock music that is noticeably influenced by classical music.

- Rock music that does not attempt to be pop music or to manufacture "hits".

- Rock music that is heavily focused around the instrumental aspect, for example, having extended instrumental sections on the albums.

- Rock music that is clearly written for the purpose of amusing the people who wrote and performed the music, rather than a blatant attempt to appeal to the public.

- Rock music with extensive production and complex arrangements.

- Rock music that is frequently experimental and non-conventional.

- Because of all of the above, it is often appreciated mostly by other musicians. I think the best summary term would be "music for musicians".

I don't know where the word came from, but most bands who get labeled as "progressive" are often known to have their style and sound change progressively as their careers progress instead of just re-making the same albums over and over again, so perhaps that's where it came from.

Bands that I feel deserve this name:

Yes
Rush
Jethro Tull
Dream Theater
Queensryche
Queen
Genesis

I don't think this category really existed much before the '70's. I'm not sure if Led Zeppelin and The Who fall into this category or not. The Beatles, in their time, were probably the most progressive band that was successful and well-known, but they break some of the guidelines on my list and I think they predate the term.

/me goes to look on the web for other definitions to see if they fit mine.