Star Trek has a federation of Humans, Vulcans, and many other races. Their ships are still separate, but they work together.
The Fediverse has a federation of Startrek.Website, Lemmy.World, Sh.itjustwo.rks, etc.
Email has a federation of gmail.com, yahoo.com, microsoft.com, aol.com, every corporate.com, spectrum.net, tons and tons of small servers. This is probably the biggest federation. Many servers will choose to only federate with specific instances, or may explicitly defederate with a list of instances. If I stand up a private, home email on my own domain, there’s no guarantee that your work will accept my emails.