There is of course no clear line between atmosphere and no atmosphere, it's continuous reduction in pressure. The usual demarcation used is Karman line. As altitude increases and pressure decreases, the velocity required for fixed wing craft to stay aloft increases, Karman line is altitude where speed of of an hypothetical ideal aircraft would have to match orbital speed to stay flying, which of course would make it a spacecraft instead. So that is considered the end of aeronautics and start of astronautics.

And yeah it fluctuates, mostly depending on solar wind conditions. So actual used altitudes are convenient round numbers, 50 miles by US and 100km by rest of the world.