De facto is Latin for "of fact," meaning "in reality," and it's usually contrasted with "de jure," which means "of law," or "officially."
If you're the de facto mayor of your town, you’re acting as mayor, even though you weren't legally elected. You may be just helping out while the official mayor — the de jure mayor, the person who was officially elected — is out of town or ill. Or it might be more sinister — you seized power from the true mayor and locked him in the basement of town hall, and now you’ve made yourself the de facto mayor.
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