A geographical region effectively dominated by two or more major powers, and open to military and political annexation by any of them at will, but left intentionally unclaimed, in the interest of providing a neutral "cushion" between the great powers.
Such a construction is often the mark of an uneasy peace - and of recognition that a shared border is an invitation to border incidents which might conceivably escalate into outright war. Under the circumstances, the major powers choose to divest themselves of formal control of the region, in order to secure stability.
Related concepts: no man's land, buffer state.
Example: Wakhan.