Where does the atmosphere end and outer space begin?

Actually Earth is also part of space. However, by the term “space” it is mostly meant the part of space outside the Earth and its atmosphere.

The outermost layer of the atmosphere ends at approximately 960 kilometers above the surface of the Earth. Even at an altitude of 1000 kilometers, there is a cloud layer of hydrogen atoms around the Earth. However, the density of the outermost layer of the atmosphere is very low. That's why it's hard to say where it ends.

According to the definition made by the Fédération Aéronautique Internationale (FAI), it is assumed that space starts at an altitude of about 100 kilometers from sea level. This level, known as the Karman border, is defined as "the height at which the density of the atmosphere is too low to allow an aircraft to stay in the air under the influence of aerodynamic forces." Aerodynamic forces caused by weather resistance at altitudes above this limit have no effect on the aircraft's stay and control. An object above the Karman border must move at very high speed in order to stay in the air. This speed, which the spacecraft must have in order to move in orbit stably, is called the orbital speed.