Definition
A flight conducted under Instrument Flight Rules, in which the pilot operates the aircraft by reference to cockpit instruments rather than outside visual cues, and navigates and is separated from other traffic under the direction of Air Traffic Control. IFR flight requires an instrument rating, a properly equipped aircraft, and an ATC clearance, and may be conducted in either instrument or visual meteorological conditions.
Plain English
A flight where the pilot flies by the instruments instead of by looking outside, follows ATC instructions the whole way, and operates under a stricter set of rules that allow flight in clouds and low visibility.
Context Anchor
Seen when comparing visual and instrument navigation, planning a route, checking weather, or operating with air traffic control on an instrument flight.
Derivation
IFR stands for Instrument Flight Rules. The phrase 'flight under IFR' simply means a flight being conducted while subject to that ruleset, much like 'driving under the speed limit' means driving while subject to a speed limit.
Why Pilots Care
Permits continued flight and arrival when clouds or low visibility prevent safe visual navigation, provided the aircraft and pilot meet regulatory requirements.
Intuition Check
Do not assume flight under IFR means the airplane must be in clouds. It means the flight is being conducted under instrument flight rules, and that can happen in clear weather too.
Example Sentence 1
Because a layer of clouds covered the route at 4,000 feet, the pilot filed and conducted the trip as a flight under IFR.
Example Sentence 2
After takeoff the crew transitioned to flight under IFR and received radar vectors from the controller.