This thread at least implies a basic definition of the Fediverse: Fediverse = ActivityPub.
But there's more than "uses only ActivityPub and ActivityPub all the time" and "doesn't understand ActivityPub".
For example, there is (streams). It can speak ActivityPub. But it is not based on ActivityPub. It's based on Nomad. It also speaks Zot6. And its ActivityPub support is optional, both server-wide and per channel, although it's on by default in both places.
And then there's (streams)' big ancestor, Hubzilla. It, too, can speak ActivityPub. But it is not based on ActivityPub either. It is based on Zot6. It can speak a whole lot of protocols. ActivityPub support is established by an add-on named PubCrawl which, by default, is activated at server level, but deactivated on newly created channels.
Do both count as parts of the Fediverse because they both have ActivityPub support in some way?
Do both not count as parts of the Fediverse because neither of them is based on ActivityPub?
Do both not count as parts of the Fediverse because ActivityPub is optional at server level? Or because it's optional at channel level?
Does (streams) count as part of the Fediverse because new channels support ActivityPub by default, but does Hubzilla not count as part of the Fediverse because new channels don't support ActivityPub by default?
Does (streams) count as part of the Fediverse because ActivityPub support is built into the core, and does Hubzilla not count as part of the Fediverse because ActivityPub support comes from an add-on? (Side-effect, by the way: (streams)' directory lists actors using ActivityPub, Zot6 and/or Nomad, but Hubzilla's directory only lists channels using Zot6 in some way.)
Or would you draw the line around individual servers, i.e. Hubzilla hubs and (streams) servers do not count as part of the Fediverse if the admins have turned ActivityPub off server-wide?
Or would you go as far as drawing the line around individual channels, i.e. your (streams) channel officially leaves the Fediverse when you turn ActivityPub off, or your Hubzilla channel won't be part of the Fediverse until you install PubCrawl?
I think there's two concurrent, but different definitions of the word Fediverse. One means, software that can speak the ActivityPub protocol. And the other one means, social media service which is able to interconnect between different websites.
The first one is more useful if you want to use it and know whether it connects you to your friends on Mastodon and the other big ones. The latter is the more technical definition and includes older protocols as well, as well as newer ones and alternative approaches to form a network in a certain way. I guess it's the more correct one. But it doesn't tell you a lot as a user. Maybe technically it can exchange your user statuses but nobody uses it so you can't really do anything with it in reality. Or there's two approaches and you were talking about a different manifestation than somebody else, and you're both federated but not part of any compatible ecosystem.