All flights that are flown under 'instrument flight rules', i.e. under the control of an air traffic controller.
Other flights are flown under VFR : 'visual flight rules' (VFR).
(Eurocontrol)
A set of regulations and procedures for flying aircraft without the assumption that pilots will be able to see and avoid obstacles, terrain, and other air traffic.
The most important concept of IFR flying is that it allows continued flight operations in reduced visibility, during which time the ability of a pilot to physically see and avoid collision with other aircraft or obstacles is severely reduced, or even impossible.
The distance that is achieved when avoiding obstacles or other aircraft is termed separation.
In controlled airspace, Air Traffic Control (ATC) separates IFR aircraft from obstacles and other IFR aircraft by applying separation rules based on time, distance, and altitude differences between aircraft, by relying either on radar or reports of aircraft positions (traditionally sent as voice radio transmissions, but increasingly as electronic data exchanges).
=> Glossary