Occupatio

Occupatio was an original method of acquiring ownership of un-owned property ("res nullius") by occupying with intent to own. 
Roman legal writings on acquisition by "occupatio" (occupation).
Nicholas argues this is the "archetype" of all other Roman law methods of original acquisition. According to the Roman jurist Gaius, any previously unowned thing becomes the just property of the first occupant able to "capture" it:
Abandoned goods ("res derelictae") was also "res nullius" and subject to acquirement through "occupatio". Land, however, was excluded and could not be acquired using "occupatio". 
Occupatio in the modern world.
The Roman law "Occupatio" has continued relevance in present times, partly due to its adoption by legal systems across Europe, Africa and North America. It is also used in international law. 
Domestic legal systems.
Legal systems across the modern world continue to employ a form of "occupatio". A full discussion of each legal system is outwith the discussion of this article but see: South African property law, civilian property law, Scots Law, Occupatio (Scots law). 
International law.
International law adopts much of Roman property law in regards to acquisition of sovereignty due to the European nature of early European discovery voyages such as Christopher Columbus. 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). An example of occupatio under international law is the United Kingdom's acquisition of ownership of Rockall in the North Atlantic Ocean by the Island of Rockall Act 1972. 