Instrument Flight Rules

Instrument flight rules (IFR) is one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other is visual flight rules (VFR). ... It is also a term used by pilots and controllers to indicate the type of flight plan an aircraft is flying, such as an IFR or VFR flight plan.

When operation of an aircraft under VFR is not safe, because the visual cues outside the aircraft are obscured by weather or darkness, instrument flight rules must be used instead.