What is Res Communis?

Legal Definition
Res communis is a Latin term derived from Roman law that preceded today’s concepts of public domain and common heritage of mankind. It has relevance in international law and common law.

In sixth century C.E., the Institutes of Justinian codified the relevant Roman law as: “By the law of nature these things are common to mankind - the air, running water, the sea, and consequently the shores of the sea.”

Biological examples of res communis include fish and mammals in high seas. Rules for use of the continent Antarctica were based on res communis as was development of space law.

The term can be contrasted with res nullius, the concept of ownerless property, associated for example with terra nullius, the concept of unowned property by which British settlement in Australia was based, despite being home to indigenous peoples. Under international law, terra nullius is a territory not under any state.
-- Wikipedia