One bit of fediverse jargon that bandied about without a lot of explanation is "instance".
I am pretty sure we started using it at StatusNet to talk about installations of our software. I am pretty sure it came from the OOP-y and/or POSIX-y sense of "instance", like "an instance of a class" or "a running instance of the program":
https://en.wikipedia.org/wiki/Instance_(computer_science)
It's kind of weird that it's become a term meaning "a domain" or "an account server" or "a community."