Which body formally adopted the 100 km mark as the universal standard defining the start of space?

Fédération Aéronautique Internationale (FAI)

The Fédération Aéronautique Internationale (FAI), which functions as the global authority for governing aeronautical and astronautical records, was responsible for formally establishing the 100-kilometer altitude as the universal benchmark for the beginning of space. This decision was made in the 1960s to provide a consistent and necessary reference point for categorizing accomplishments in flight and space exploration across different nations. While this standard is internationally accepted for record-keeping and general definition, its adoption was based on consensus rather than being derived from a unique, sharp physical phenomenon indicating the absolute end of the atmosphere. This means the FAI's definition carries historical and regulatory weight within its jurisdiction, even if pure scientific metrics might suggest a slightly different physical transition point.