er.. dude, I don't want to come across as being an asshole, but you've got totally the wrong end of the stick. Progressive Metal, are the bands he mentioned, eg Dream Theater, etc.. Progressive Rock is stuff like Pink Floyd, Yes, Rush, etc.. Search the internet, its all true.
You are right though, Opeth is progressive metal BECAUSE its influenced by different genres, IN THIS CASE black/death metal and folk or whatever, but that doesn't work specifically the other way round. Its progressive because its a mixture of genres.. doesn't matter what genres they are. The definition of prog music, specifically, certainly has nothing to do with black or death metal. And no, i'm not saying that the definition of prog rock/metal is a mixture of genres, i'm just trying to relate it to what you are thinking
As for what the term 'progressive' actually means, well, thats anyones guess.! I think the essense of its meaning in a musical sense is the word 'Different'. It doesn't fit into the mold of the genre etc.. therefore any genre can be labelled progressive, and perhaps the only reason progressive rock came first, is because rock came first! Just my thoughts!