Is it any service that includes the ability to federate? If so, are Matrix/XMPP/other non-ActivityPub services part of the Fediverse, or does Fediverse really just mean ActivityPub?
The context is wanting to promote a Matrix chat room on discuss.online: #online.discuss:discuss.online (see post). Does it belong in Lemmy communities like this one, !fedigrow@lemm.ee, etc? Or does it not count as part of the Fediverse?
According to https://en.wikipedia.org/wiki/Fediverse:
The majority of Fediverse platforms are based on free and open-source software, and create connections between servers using the ActivityPub protocol. Some software still supports older federation protocols as well, such as OStatus, the Diaspora protocol and Zot. Diaspora* is the only actively developed software project classified under the original definition of Fediverse that does not support ActivityPub.[5][6]
You would have to look at those citations to see how authoritative they are. This may also still be open to interpretation?