The term “no man's land” is used to describe unoccupied land which is positioned between two or more military forces. The term is also sometimes used more generally to describe land without a clear provenance or ownership, or land which has remained unclaimed for one reason or another. However, most people associate no man's land specifically with its military sense, and the term is especially widely used in reference to the First World War, when the trench warfare system created an abundance of unoccupied land.
The use of this term dates back to at least the 1400s, when Londoners referred to a patch of land north of the city used for executions as “no man's land.” The implication was that no person would want to attempt to claim the land, since it was often covered in gibbeted bodies and other ghastly sights, and because it had such a brutal history. In the military sense, this phrase emerged around the 1900s.
In classic trench warfare, no man's land separates the enemy trenches, and the space can vary considerably. Soldiers may periodically be sent “over the top” to try and push across this area to attack the enemy, with the goal of gaining control of their trenches, and thereby gaining ground. As a result, a new no man's land will be created, between the occupied enemy trenches and those still controlled by the enemy.
In modern warfare, a patch of no man's land between enemy emplacements is very common, and it often appears on contested borders, as well. When two countries are in conflict about their borders or have differing ideas about border security, a space may emerge between two border checkpoints, which means that people crossing the border pass through an area of unoccupied land which acts almost as a buffer zone.
No man's land is often a forbidding place. In warfare, it is typically littered with unexploded ordinance, mines, broken military equipment, and other detritus. Historically, dead bodies were often found in no man's land as well, serving as grisly reminders of the fate of previous sorties across the space. Sometimes, no man's land becomes a haven for wildlife and plants; a huge stretch between North and South Korea, for example, has become a de facto nature preserve, because few people venture into it.
People enter no man's land to gather intelligence about the enemy, collect the dead, or to attempt to take territory. Typically, such ventures are very dangerous, and most military personnel do not relish orders which cause them to enter this area as a result.