The Kármán line is defined as the limit between atmosphere and outer space, for the purposes of aviation and astronautics. This definition is accepted by the International Aeronautical Federation, which is an organization dedicated to establishing international standards and recognizing records in aeronautics and astronautics.

Its height was estimated at 100 km above sea level. It is also obtained by calculating the height at which the density of the atmosphere becomes so low that the speed of an aircraft to achieve aerodynamic lift through wings "Wing (aeronautical)") and propellers "Propeller (device)") should be comparable to the orbital speed for that same height, so that once that height is reached by these means the wings would no longer be valid to maintain the ship.

The Kármán line is named after the Hungarian-American Jewish physicist and engineer Theodore von Kármán.

The atmosphere does not end abruptly at a given height, but rather becomes increasingly tenuous with altitude. Also, depending on how the different layers that make up the space around the Earth are defined (and depending on whether these layers are considered part of the actual atmosphere), the definition of the edge of space varies considerably. If the thermosphere and exosphere had to be considered part of the atmosphere and not space, we would have to expand the concept of atmosphere to some above sea level. Thus, the Kármán line is an arbitrary definition based on the following considerations:

An airplane is only lifted if it is constantly moving in the air (the speed of travel in the air does not depend on the speed with respect to the Earth), so the wings generate lift. The thinner the air, the faster the plane has to go to generate enough lift to not fall; The lift coefficient of a wing for a given angle of attack is known (or is estimated by various methods). An object only remains in orbit if the centrifugal component of its acceleration is sufficient to offset the "downward" pull of gravity. If the horizontal displacement speed decreases, its centrifugal component also decreases, and gravity will cause its altitude to decrease.

The speed required for this balance is called orbital speed, and it varies with the height of the orbit. For the International Space Station or a space shuttle in low Earth orbit, the orbital speed is about 27,000 kilometers per hour (17,000 miles per hour). For a plane that is trying to fly higher and higher, the air becomes less dense, and that forces the plane to increase speed to create enough lift. The Kármán line is a concept primarily related to altitude, and consequently to the need to travel at a certain speed in order to obtain aerodynamic lift or, in any case, compensation for the gravitational pull. In practice, these considerations vary as the radius of the orbit increases, because the larger the radius, the less gravitational pull, and the less centrifugal acceleration for the same linear speed. However, the definition of the Kármán line neglects this effect due to orbital velocity, so it would be sufficient to maintain any altitude, regardless of atmospheric density. Therefore, the Kármán line is the lowest altitude at which orbital speed alone can provide sufficient compensation for gravitational attraction, or, conversely, the highest altitude at which the displacement of an object causes it to obtain lift from the atmosphere if it exceeds a certain speed.