Yeah, the meaning of Progressive is rather indefinite when it comes to music.

The dictionary meaning of progressive could probably be applied to any new music that is worth buying. Or put another way, anything that isn't progressing the musical arts probably isn't worth buying.

But I would say that those bands who are collectively thought of as 'prog' do tend to try more varied musical ideas than those who aren't.

Unfortunately, into this pigeon-hole also drop those bands that think an interesting CD consists purely of near-identical 15 minute long overly-theatrical stories of middle-age heroics, punctuated by heavy church-organ keyboard riffs, and as many key, tempo and time signature changes as they can cram in.

I never liked the term - it's too broad and vague.