The territory, domain, or jurisdiction ruled by a baron; also the rank or dignity of being a baron.
From baron (Old French, from Medieval Latin baro) plus -y, a suffix forming abstract nouns and place names. The -y creates both the abstract concept (the state of being a baron) and the concrete place (the baron's lands).
Barony shows off English's flexibility—the same word can mean both a place (the barony of Cornwall) and a condition (the barony is hereditary), and we understand both from context without confusion.
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