Signature themes
 

It has escaped me but I'm sure there's a word for a signature theme or prominent song on an album by a prog band. I know the songs but I can't remember the term, some examples:

Karn Evil 9 by ELP
Nine Feet Underground by Caravan
Lady Fantasy by Camel
2112 by Rush
In the Court of the Crimson King by King Crimson
Achilles Last Stand by Led Zeppelin
Suppers Ready by Genesis

Kind of Prog thematic or Symphonic...feature length songs. More of these kind of songs would be good, too.

Epic multi-part suite?

Yeah, that must be it thanks man. Its usually the same as the album name, not always in parts but sometimes. Typified by long lengths and skilful expressive playing. If it's anything it has to be one of those words so I think I meant Suite all along if not all three.

Masthead?

I usually just call it main theme or reprise.

Very obvious example