My understanding is that anything that is using activity pub to communicate is the fediverse.
Software, instance, or channel that isn't actively communicating via activity pub, is not the fediverse.
Software, instance, or channel that is actively using activity pub to communicate (even if they are using other methods too), is the fediverse.
I've never read the activity pub documentation, so maybe there is more to it than that, but the above is my current understanding.
They'd also have a very contentious discussion community.
!yepowertrippinbastards@lemmy.dbzer0.com
Edit: fixed link.
Edit 2: for clarity, I'm saying the topic is contentious (going to get very opinionated points of view), not saying anything about the mods/admins/users.