A characteristic belonging to a thing. Also what is owned — the same word for what a thing is and what is held.
Latin proprietas (ownership, a peculiar or special quality), from proprius (one's own); the same word came to mean both a characteristic a thing has and a possession a person holds, because both are things marked as belonging.
A characteristic belonging to a thing, and also what is owned by someone — the root makes no distinction between them, because both are a form of belonging. A property, in either sense, names what is proper to something.