Federated is a bit different from decentralised. For example, email is federated, but not decentralised. In a federated network, if "your service providing server" is down (like gmail.com), then you don't have service, though others on other parts of the network would continue to have service. With a decent sized network, there is no "your service providing server" and other machines can take over when one fails. Decentralisation always goes along with redundancy whereas federation doesn't require redundancy.
You're thinking of distributed. Decentralized is broader than that. Really anything that isn't centralized like Facebook is.
Obviously a lot of this is very squishy and overlaps a lot even within the same system.
Look at DNS. It's nominally a centrally-rooted hierarchy, but the root is operated by consensus rather than monarchy, so how do you classify that? Then each domain can be operated by a separate organization, so essentially federated but not exactly because it's still a hierarchy. Meanwhile the recursive resolvers and caches are fully distributed -- use any of them and the results are (supposed to be) the same.
(General note)
Federated is a bit different from decentralised. For example, email is federated, but not decentralised. In a federated network, if "your service providing server" is down (like gmail.com), then you don't have service, though others on other parts of the network would continue to have service. With a decent sized network, there is no "your service providing server" and other machines can take over when one fails. Decentralisation always goes along with redundancy whereas federation doesn't require redundancy.