ActivityPub
ActivityPub is an open, decentralized social networking protocol based on Pump.io's ActivityPump protocol.[1] It provides a client/server API for creating, updating, and deleting content, as well as a federated server-to-server API for delivering notifications and content.[2]
The fediverse is an ensemble of federated (i.e. interconnected) servers that are used for web publishing and file hosting, which, while independently hosted, can communicate with each other.
On different servers , users can create so-called identities. These identities can communicate across the boundaries of the instances because the software running on the servers supports one or more communication protocols that follow an open standard.