Occupatio (occupation) was an original method of acquiring ownership of un-owned property (res nullius) by occupying with intent to own.
[1] According to the Roman jurist Gaius, any previously unowned thing becomes the just property of the first occupant able to "capture" it:[2] § 66.
For wild beasts, birds, and fishes, as soon as they are captured, become, by natural law, the property of the captor, but only continue such so long as they continue in his power; after breaking from his custody and recovering their natural liberty, they may become the property of the next occupant; for the ownership of the first captor is terminated.
[3] The Roman law occupatio has continued relevance in present times, partly due to its adoption by legal systems across Europe, Africa and North America.
[4] Occupatio was later employed under public international law as the basis of acquisition of states ownership of vacant territory (often including land already possessed by indigenous populations).