Plain English Meaning
A flight plan filed with air traffic control that says the pilot will fly by instruments and follow ATC instructions for the whole flight. Once activated, ATC keeps the aircraft separated from other traffic and guides it along the route.
Full Definition
A formal flight plan filed with air traffic control under Instrument Flight Rules, specifying the proposed route, altitude, airspeed, departure and destination airports, estimated times, and other required details, under which the flight will be conducted with ATC clearances and separation services regardless of weather conditions.