Tbf ActivityPub is the name of the protocol, and Fediverse is the name of the network, though is too generic so many of us have taken to calling us the "Threadiverse", for threaded-based conversational format (Lemmy + Mbin + PieFed + nodeBB + flarum), as distinct from the user-centric Mastodon, the image-based Pixelfed, video-based Loops (not federated yet iirc), etc. Notably though, while it is not so easy, people can send messages between these e.g. from Mastodon to Lemmy (or PieFed) and vice versa.