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.
Plain English
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.
Context Anchor
You encounter an IFR flight plan before an instrument flight, during preflight planning, when getting a clearance, and in navigation discussions about flying a planned route.
Derivation
IFR stands for Instrument Flight Rules. A 'flight plan' is the document submitted to ATC describing the intended flight. Together, the term means the planned flight will be conducted under the rules that require flying primarily by reference to instruments and under ATC control.
Why Pilots Care
It is required to receive ATC navigation services and traffic separation in instrument conditions or in controlled airspace under IFR.
Intuition Check
Do not read “plan” as just your private idea of where you want to go. An IFR flight plan is a formal filing, and it still must be followed by an IFR clearance before flying IFR in controlled airspace.
Example Sentence 1
Because the forecast called for low ceilings along the route, the pilot filed an IFR flight plan before departure.
Example Sentence 2
ATC amended the IFR flight plan in flight to route the aircraft around a temporary restricted area.