Fediverse

The Fediverse is the name given to a group of computer servers that host social networks in a peer-to-peer way. Each of these servers are able to talk with each other to make one gigantic web of social networks. This is similar to if Twitter users could see posts from Facebook users, and Facebook users could see posts from Twitter users. Its name is a combination of the words "federation" and "universe". 
Most instances of the Fediverse make use of free, open source software. They also use technology standards called ActivityPub and OStatus. To contribute content to the Fediverse you can either host your own server instance, or join an existing one as a user. Within the fediverse, there is not "one" federation, but rather numerous "bubbles of federation", more or less isolated or interconnected.