Federated = Email. Different email providers (Gmail, Microsoft, iCloud, etc.) can all communicate with each other freely and using the same protocol.

Defederated = No longer federated.

Not federated = Your company’s internal Teams/Slack/whatever messaging system. Even tho they theoretically could communicate without other messengers and/or people outside of the company, they typically don’t do they’re just their own little bubble.

Lemmy is federated, Facebook is not.