Is this actually a well-defined altitude, or does it depend on the properties of the vehicle? (Certainly, the minimum speed to maintain level flight normally does.) The Karman line is near but distinct from the boundary between the thermosphere and the exosphere, which is well-defined: it's where atoms become so thin that their mean free path becomes so long they stop bouncing off each other and just follow parabolic trajectories under gravity.

This description made no sense to me until I read this from the same page:

> The Karman line is therefore the highest altitude at which orbital speed provides sufficient aerodynamic lift to fly in a straight line that doesn't follow the curvature of the Earth's surface.