The Kármán line - or the von Kármán line - is a somewhat arbitrary definition of the boundary between the Earth's atmosphere and space. It is named for Theodore von Kármán, a founder of CalTech's Jet Propulsion Laboratory, who worked to define it in 1957. His original calculations gave him an altitude of approximately 84 kilometers, but he felt that 100 km would be easier for everyone to remember, so he defined it as 100 km (62 statute miles if you're metrics-challenged).
This definition of the boundary of space was adopted by the FAI (Fédération Aéronautique Internationale), and as such, most non-US Government space programs use it to determine the boundary. The US Government generally follows the USAF/NASA definition of 50 statute miles.