I’m gonna take a step back and introduce a more commonly heard/understood term. According to Merriam-Webster, a FEDERATION is:
an encompassing political or societal entity formed by uniting smaller or more localized entities: such as
a. a federal government
b. a union of organizations
Definition 1b. best sums up a federated system in the sense of OP’s question. It’s a collection of smaller independent organizations united for a particular cause. In the case of Lemmy and other federated sites, sharing digital information. No single federated site controls all of the “fediverse,” yet they can all technically communicate with each other as if they’re one entity.
Defederated, on the other hand, is just the opposite. A single centralized organization with complete control over the content they provide. Reddit, being one corporation that has complete control over all content on their own servers, is a defederated site.