Definition
Weather conditions expressed in terms of visibility, distance from clouds, and ceiling that are less than the minimums specified for visual flight rules (VFR), requiring the pilot to fly by sole reference to the aircraft instruments rather than outside visual cues.
Plain English
Weather bad enough that you can't see well enough outside the aircraft to fly visually, so you have to fly using only what the cockpit instruments are telling you.
Context Anchor
You will see IMC in weather briefings, flight planning, navigation discussions, and warnings about losing outside visual reference in flight.
Derivation
From 'instrument' (the cockpit gauges used for reference), 'meteorological' (relating to weather), and 'conditions' (the state of the atmosphere). The phrase names exactly what it describes: weather that forces you to rely on instruments.
Why Pilots Care
Flight in IMC without an instrument rating, current experience, and an IFR clearance is illegal and a leading cause of fatal accidents due to spatial disorientation.
Grounding Statement
Picture flying into a cloud: the horizon and ground disappear, and the instruments become the pilot’s main source of attitude, altitude, heading, and navigation information.
Intuition Check
IMC does not mean simply “using instruments.” It means the outside weather no longer provides enough visual reference for normal visual flying.
Example Sentence 1
The ceiling dropped to 600 feet and visibility fell to one mile, so the flight was now in IMC and required an instrument-rated pilot.
Example Sentence 2
The weather briefing indicated IMC would develop after sunset, so the pilot filed an IFR flight plan.