[zrm]

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.