It's miner and user confidence in a specific chain that ultimately defines it. Similar to country sovereignty, these things are defined only by how most people

It's miner and user confidence in a specific chain that ultimately defines it. Similar to country sovereignty, these things are defined only by how most people define it. You can't just stand up and say you are now your own country, you need to get other countries, other people, to agree with you. But at the same time, if a country changes its name/laws/maps, it doesn't become a "new country", it just changes. You can fork bitcoin, but unless most users and miners out there agree to call your fork "bitcoin", then it won't be bitcoin. But if "bitcoin" decides to radically change, and most people agree with the change, then it is still bitcoin, despite the differences. (as an aside, anywhere I say "most people" here, you can replace that with "most hash power" or "most users" or any other metric you want. I'm not trying to speak to specifics, but more the overall idea)