a very broad definition would include all of these social networks - most of which are incompatible - but in the most common sense these days, people mean anything ActivityPub compatible, with Mastodon being the most popular server software for that. i suspect Nostr may have taken some inspirations and lessons from it, but wanted to make it more decentralised. on Mastodon, you're still relying on making accounts on specific servers. the ability to migrate identities was added a while back, but you can't bring your posts with you, so it's still a bit limited.