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 under Instrument Flight Rules (IFR) by a properly rated pilot in a suitably equipped aircraft.
Plain English
Weather that is too poor to fly by looking outside. The visibility is too low, the clouds are too close, or the cloud base is too low to safely judge attitude and separation by eye, so the pilot must fly using the aircraft's instruments.
Context Anchor
Pilots encounter this term in weather briefings, flight planning, flight rules decisions, and reports such as entering or leaving Instrument Meteorological Conditions.
Derivation
Instrument refers to flying by reference to the cockpit instruments rather than by looking outside. Meteorological comes from the Greek meteoron, meaning something high in the sky, and is the root of meteorology, the study of weather. Together the phrase names weather conditions that force the pilot to rely on instruments.
Why Pilots Care
Crossing into these conditions without an instrument rating or IFR clearance violates regulations and quickly leads to loss of control or controlled flight into terrain.
Grounding Statement
If clouds are too low or visibility is too poor to maintain the required outside visual references, the flight is in Instrument Meteorological Conditions.
Intuition Check
Instrument Meteorological Conditions does not mean any time a pilot looks at cockpit instruments. It means the weather itself is below the visual-weather limits.
Example Sentence 1
The ceiling dropped to 600 feet overcast and visibility fell to two miles, so conditions were now IMC and only IFR-rated pilots could legally depart.
Example Sentence 2
After encountering instrument meteorological conditions en route, the crew requested an IFR clearance and began flying solely by reference to instruments.