Terra nullius

Terra nullius means unclaimed territory. It is the correct term in international law. It means land which does not (at present) belong to any state. The term comes from the Latin, meaning "Nobody's thing". There is also an expression "no man's land", but that does not have a formal legal meaning.
Rule over "terra nullius" can be got by occupying the territory. There are various legal problems about this concept, but in practice it is often the business of neighbouring states to decide by negotiation. Diplomacy may sometimes be replaced by warfare.
Two territories that are currently "terra nullius" are Bir Tawil between Egypt and Sudan, and Marie Byrd Land, in Antarctica.