>the action of preventing part or the whole of a book, film, work of art, document, or other kind of communication from being seen or made available to the public, because it is considered to be offensive or harmful, or because it contains information that someone wishes to keep secret, often for political reasons

>a system in which an authority limits the ideas that people are allowed to express and prevents books, films, works of art, documents, or other kinds of communication from being seen or made available to the public, because they include or support certain ideas:

These are from Cambridge. They don't really apply to a federated social media either: nothing is prevented to be seen, people can just choose not to see it if they want.