The Kármán line (or von Kármán line /vɒn ˈkɑːrmɑːn/) is a conventional definition of the edge of space; it is widely but not universally accepted. The international record-keeping body FAI (Fédération aéronautique internationale) defines the Kármán line at an altitude of 100 kilometres (54 nautical miles; 62 miles; 328,084 feet) above mean sea level.

While named after Theodore von Kármán, who calculated a theoretical limit of altitude for aeroplane flight at 83.8 km (52.1 mi) above Earth, the later-established Kármán line is more general and has no distinct physical significance, in that there is a rather gradual difference between the characteristics of the atmosphere at the line, and experts disagree on defining a distinct boundary where the atmosphere ends and space begins. It lies well above the altitude reachable by conventional airplanes or high-altitude balloons, and it is approximately where satellites, even on very eccentric trajectories, will decay before completing a single orbit.

The Kármán line is mainly used for legal and regulatory purposes of differentiating between aircraft and spacecraft, which are then subject to different jurisdictions and legislations. While international law does not define the edge of space, or the limit of national airspace, most international organizations and regulatory agencies (including the United Nations) accept the FAI's Kármán line definition or something close to it. As defined by the FAI, the Kármán line was established in the 1960s. Various countries and entities define space's boundary differently for various purposes.

The FAI uses the term Kármán line to define the boundary between aeronautics and astronautics:

The expressions "edge of space" or "near space" are often used (by, for instance, the FAI in some of their publications) to refer to a region below the boundary of Outer Space, which is often meant to include substantially lower regions as well. Thus, certain balloon or airplane flights might be described as "reaching the edge of space". In such statements, "reaching the edge of space" merely refers to going higher than average aeronautical vehicles commonly would.

There is still no international legal definition of the demarcation between a country's air space and outer space. In 1963, Andrew G. Haley discussed the Kármán line in his book Space Law and Government. In a chapter on the limits of national sovereignty, he made a survey of major writers' opinions. He indicated the inherent imprecision of the Line:

In essence, the line represents a mean or "median" measurement. It is comparable to such measures used in the law as "mean sea level", "meander line", "tide line"; but it is more complex than these. In arriving at the von Kármán jurisdictional line, myriad factors must be considered – other than the factor of aerodynamic lift. These factors have been discussed in a very large body of literature and by a score or more of commentators. They include the physical constitution of the air; the biological and physiological viability; and still other factors which logically join to establish a point at which air no longer exists and at which airspace ends.

In the final chapter of his autobiography, Kármán addresses the issue of the edge of outer space: