A federation is a group of organizations that each have some autonomy but are subject to some extent to the group’s centralized control. That makes it partially decentralized.
The Fediverse is a federation of interconnecting computer servers facilitating social interactions. In this context, the centralized control is often limited to the co-development and adoption of the protocols necessary for interoperability. This means the Fediverse can be considered a decentralized network assuming there is no advantage to everyone piling onto the same server and so less risk of anyone centralizing protocol governance.
The servers are peers in the distributed computing sense. Nevertheless, the Fediverse constitutes a client-server system (the computer resources used by each user are not equal to the servers) and so does not constitute a peer-to-peer system.
Popular protocols include ActivityPub and Diaspora.
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