Federation
Federation refers to a network of independent entities that co-operate to provide a common service to its users. In a federated system, each entity maintains control over its own operations and resources, but also agrees to abide by certain common protocols or standards to enable collaboration and interoperability. With such interoperability, allows users of each entity to interact with each other, as if they were all part of one big entity.
An example of federation is email.
Each email provider is its own independent service, but each operating under commonly agreed-upon set of procedures in order to allow its users to send messages to users of other entirely independent email service providers.
Another more ostensible example of federation is the Fediverse, more specifically, software that are compatible with WebFinger and ActivityPub.
This runs counter to a more centralized system, where if a person desires to communicate with another person, both will need to hold accounts under the same service.
Federation falls under the umbrella of "decentralization".