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?

So, the term Fediverse basically refers to the ActivityPub protocol and the associated server software. Technically, it's not just ActivityPub, but also the AT protocol and nostr. Another layer to the Fediverse, at least in my mind, is having some federation. Meaning Bluesky isn't apart of the Fediverse, even though it's built on the AT Protocol, because of it's isolation. Since there's no interaction with Lemmy, Mastodon, PixelFed, or other federated social networks, it's not part of the Fediverse.