Separation

In air traffic control, separation is the name for the concept of keeping an aircraft in a minimum distance from another aircraft to reduce the risk of those aircraft colliding, as well as prevent accidents due to wake turbulence.
Air traffic controllers apply rules, known as separation minima to do this. Pairs of aircraft to which these rules have been successfully applied are said to be separated: the risk of these aircraft colliding is therefore remote. If separation is lost between two aircraft, they are said to be in a conflict.

Vertical separation

Between the surface and an altitude of 29,000 feet (8,800 m), no aircraft should come closer vertically than 300 metres or 1,000 feet (in those countries that express altitude in feet), unless some form of horizontal separation is provided. Above 29,000 feet (8,800 m) no aircraft shall come closer than 600 m (or 2,000 feet), except in airspace where Reduced Vertical Separation Minima (RVSM) can be applied.

Horizontal separation

If any two aircraft are separated by less than the vertical separation minimum, then some form of horizontal separation must exist.