IFR - 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).
Instrument flight rules are rules and regulations to govern flight under conditions in which flight by outside visual reference is not safe. IFR flight depends upon flying by reference to instruments in the flight deck, and navigation is accomplished by reference to electronic signals.
IFR page at Wikipedia website. * IFR page at SKYbrary website.