...when a platformer has a huge world and you keep revisiting the same places as new areas become open (like VVVVVV or Gateways), as opposed to one made up of single rooms/levels (like Commander Keen) or continuous linear where you just keep going forward and don't revisit the same place twice (like Prehistorik). I recall it had a specific name but for the life of me I can't remember it.

Thanks.

Metroidvania?

That's the one! I was thinking Castlevania but couldn't get the name. Thanks.

EDIT: Why can't I mark your post as solution? It appears as a question post for me. Could you make a new one?

I'm beginning to suspect Metroidvania was the correct answer, but "A new one" sounds good too.