Belonging to one and closed to others by right. Privacy is not concealment — it is authority over who may see.
Latin privatus (belonging to oneself, not public), from privare (to release, to deprive), from privus (one's own); the same root gives deprive, and both words turn on the same fact of belonging to one rather than to many.
Belonging to one and closed to others by right rather than by force. What is private is not withheld from the world so much as it is simply not the world's to begin with.