Definition
Weather conditions expressed in terms of visibility, distance from cloud, and ceiling that are less than the minimums specified for visual flight. When IMC exists, flight must be conducted by sole reference to the aircraft instruments under instrument flight rules.
Plain English
Weather that is too poor to fly by looking outside. The pilot can no longer rely on what they see through the windscreen and must fly using only the instruments on the panel.
Context Anchor
Pilots see IMC in weather reports, flight planning, instrument training, air traffic control discussions, and logbook entries.
Derivation
Instrument refers to the cockpit instruments the pilot must rely on. Meteorological comes from the Greek 'meteoron' meaning 'thing high up,' the root of meteorology — the study of the atmosphere. Conditions simply means the state of the weather at a given time. Together: weather conditions that force the pilot to fly by instruments alone.
Why Pilots Care
Flying in IMC without proper IFR certification and clearance is illegal and extremely dangerous, as it can lead to loss of control or controlled flight into terrain.
Grounding Statement
If clouds, fog, rain, or low visibility keep you from seeing enough outside to fly safely by sight, you should treat the situation as IMC.
Intuition Check
IMC does not only mean “inside a cloud.” It means the weather is below the legal or practical minimums for flying by outside visual reference.
Example Sentence 1
The pilot checked the forecast and saw that IMC was expected along the route, so an instrument flight plan was filed.
Example Sentence 2
Forecast IMC conditions meant the student pilot could not log the cross-country flight under visual flight rules.