Space starts at an altitude of 100 kilometres.
As altitude increases the atmospheric pressure decreases. The is no clear dividing line as to where the atmosphere ends and space begins. It is however generally accepted that space starts at an altitude of 100km.
To put this in perspective, geostationary orbit is 35,786km.
Anybody who flies above 50 miles (=80km) is awarded astronaut status by the US space agency.