No man's land is waste or unowned land or an uninhabited or desolate area that may be under dispute between parties who leave it unoccupied out of fear or uncertainty. The term was originally used to define a contested territory or a dumping ground for refuse between fiefdoms. It is commonly associated with World War I to describe the area of land between two enemy trench systems, not controlled by either side. The term is also used metaphorically, to refer to an ambiguous, anomalous, or indefinite area, regarding an application, situation, or jurisdiction. It has sometimes been used to name a specific place.

According to Alasdair Pinkerton, an expert in human geography at Royal Holloway, University of London, the term is first mentioned in Domesday Book (1086), to describe parcels of land that were just beyond London's city walls. The Oxford English Dictionary contains a reference to the term dating back to 1320, spelled nonesmanneslond, to describe a territory that was disputed or involved in a 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 is also applied in nautical use to a space amidships, originally between the forecastle, the living quarters of ordinary sailors, and the masts in a square-rigged vessel where various ropes, tackle, block, and other supplies were stored. In the United Kingdom, several places called No Man's Land denoted "extra-parochial spaces that were beyond the rule of the church, beyond the rule of different fiefdoms that were handed out by the king … ribbons of land between these different regimes of power".

The British Army did not widely employ the term when the Regular Army arrived in France in August 1914, soon after the outbreak of World War I. The terms used 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 1908 short story "The Point of View". Swinton used the term in war correspondence on the Western Front, with specific mention of the terms concerning 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 personnel correspondences of the members of the British Expeditionary Force (BEF).

In World War I, no man's land often ranged from several hundred yards to less than 10 metres (33 ft), in some cases. Heavily defended by machine guns, mortars, artillery, and riflemen on both sides, it was often extensively cratered by exploded shells, riddled with barbed wire, and littered with rudimentary land mines; as well as the corpses and wounded soldiers who were unable to make it through the hailstorm of projectiles, explosions, and flames. The area was sometimes contaminated by chemical weapons. It was open to fire from the opposing trenches and hard going generally slowed any attempted advance.

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 had to enter it to bring in the wounded. No man's land remained a regular feature of the battlefield until near the end of World War I when mechanised weapons (i.e., tanks and airplanes) made entrenched lines less of an obstacle.

Effects from World War I no man's lands persist today, for example at Verdun in France, where the Zone Rouge (Red Zone) contains unexploded ordnance, and is poisoned beyond habitation by arsenic, chlorine, and phosgene gas. The zone is sealed off completely and still deemed too dangerous for civilians to return: "The area is still considered to be very poisoned, so the French government planted an enormous forest of black pines, like a living sarcophagus", comments Alasdair Pinkerton, a researcher at Royal Holloway University of London, who compared the zone to the nuclear disaster site at Chernobyl, similarly encased in a "concrete sarcophagus".

During the Cold War, one example of "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 (yards) in width, containing watch towers, minefields, unexploded bombs, and other such debris. Would-be escapees from Eastern Bloc countries who successfully scaled the border fortifications could still be apprehended or shot on sight by border guards in the zone. One notable incident was the killing of Peter Fechter, shot whilst attempting to cross the Berlin Wall into West Berlin.

The U.S. Naval Base at Guantánamo Bay, Cuba is separated from Cuba proper by an area called the Cactus Curtain. In late 1961, the Cuban Army had its troops plant a 13-kilometre (8.1 mi) barrier of Opuntia cactus along the northeastern section of the 28-kilometre (17 mi) fence surrounding the base to prevent economic migrants fleeing from Cuba from resettling in the United States. This was dubbed the "Cactus Curtain", an allusion to Europe's Iron Curtain and the Bamboo Curtain in East Asia. U.S. and Cuban troops placed some 55,000 land mines across the no man's land, creating the second-largest minefield in the world, and the largest in the Americas. On 16 May 1996, President Bill Clinton ordered the U.S. land mines to be removed and replaced with motion and sound sensors to detect intruders. The Cuban government has not removed the corresponding minefield on its side of the border.^{[citation needed]}