A term used to describe a geographical region, either unclaimed by any state or strongly contested by two or more states. Usually applied to the area between warring forces along a front line. Sometimes used in situations when buffer zone might be more appropriate.
No man's land may also be no man's land because it is terra incognita, unexplored and hence unclaimed (because it is unknown). However, it is a matter of historical record that there have been many examples of states claiming as-yet-unexplored territory (excellent examples of these extremes being the Treaty of Tordesillas, or the status of East Greenland prior to 1933, or Svalbard prior to the Svalbard Treaty of 1920).
Man is by nature a political animal, and nature abhors a vacuum - a no man's land is generally a very unstable condition for territory. Sooner, or later, it is usually either claimed or conquered or both.
Related concepts: buffer state, buffer zone.
Other example(s): World War I, Wakhan.