The U.S. Air Force definition of an astronaut is a person who has flown higher than 50 miles (80 kilometres) above mean sea level, approximately the line between the mesosphere and the thermosphere.
Note that von Kármán's original notes and calculations concluded the boundary should be 270,000 ft (51 miles / 82 km), but he rounded this up to 100 km.
https://www.livescience.com/63166-ou...rman-line.html