I found my old definition when trying to get my head round it.
It’s not self evident that “unread topics” are different to new topics. Really it means watched/tracked topics that you have read but which have unread replies. Is there a succinct way of explaining all this?
From: The "new new" has been enabled on Meta - #23 by Jonathan5
(I wasn’t able to use the quote function, maybe because the topic is so old.)