I always thought of progressive rock, or progressive music in general, as music that doesn't specifically follow any form. Ie: verse, chorus, verse, bridge etc etc. Progressive music seems to flow like a movie with several parts that seemlessly change, and with each changing part a new mood would arise and the scene would shift, like in a movie.
Think of a classical piece and imagine it played by a modern day rock band, I think that would fit into the progressive genre quite nicely!
That's what I think anyway, haha.