No man's land is a term for land that is not occupied or is under dispute between parties that will not occupy it because of fear or uncertainty. The term was originally used to define a contested territory or a dumping ground for refuse between fiefdoms. It is most commonly associated with the First World War to describe the area of land between two enemy trenches that neither side wishes to openly move on or take control of due to fear of being attacked by the enemy in the process.
The Oxford English Dictionary contains a reference to the term dating back to 1320, and spelt nonesmanneslond, when the terms was used to describe a disputed territory or one over which there was legal disagreement. The same term was later used as the name for the piece of land outside the north wall of London that was assigned as the place of execution. The term was applied to a little-used area on ships called the forecastle, a place where various ropes, tackle, block, and other supplies were stored. 
The British Regular Army did not widely employ the term when they arrived in France in 1914. The terms most frequently at the start of the war, to describe the area between the trench lines, included 'between the trenches' or 'between the lines'. The term 'no man's land' was first used in a military context by soldier and historian Ernest Swinton in his short story The Point of View. Swinton used the term in war correspondence on the Western Front, with specific mention of the terms with respect to the Race to the Sea in late 1914. The Anglo-German Christmas truce of 1914 brought the term into common use, and thereafter it appeared frequently in official communiqués, newspaper reports, and per personnel correspondences of the members of the British Expeditionary Force.
In World War I, traversing no man's land was often a hellish experience for soldiers, ranging from several hundred yards to in some cases as short as 15 yards. Heavily defended by machine guns and riflemen on both sides, it was often riddled with land mines and barbed wire, as well as corpses and wounded soldiers who were not able to make it across the sea of explosions and fire. The area was usually devastated by the warfare, carnage and remains of the artillery. It was open to fire from the opposing trenches and hard going generally slowed down any attempted advance. However, not only were soldiers forced to cross no man's land when advancing, and as the case might be when retreating, but after an attack the stretcher bearers would need to go out into it to bring in the wounded.
"No Man's Land is pocketmarked like the body of foulest disease and its odour is the breath of cancer...No Man's Land under snow is like the face of the moon, chaotic, crater-ridden, uninhabitable, awful, the abode of madness.
Hideous landscapes, vile noises....everything unnatural, broken, blastered; the distortion of the dead, whose unburiable bodies sit outside the dug-outs all day, all night, the most execrable sights on earth."
The hell of the no man's land remained largely impenetrable until near the end of World War I, when tanks were able to cross it with little opposition and break the defenders in their trenches.
During the Cold War, no man's land was the territory close to the Iron Curtain. Officially the territory belonged to the Eastern Bloc countries, but over the entire Iron Curtain there were several wide tracts of uninhabited land, several hundred meters in width, containing watch towers, minefields, unexploded bombs and other such debris.
No man's land is land that is not part of any country. More precisely, it is an area that is under dispute and not occupied because of fear or uncertainty. During war (especially World War I), it is a term used for the area of land between two enemy trenches. Neither side wanted to openly move on or take control of because they feared being attacked by the enemy in the process.
It is also a term for the stretch of land between two border posts. A person is in no man's land when one exits one country at their border post and then enters the next country at their border post, usually just a few meters away.