Definition
Weather conditions in which visibility, distance from clouds, or cloud ceiling are below the minimums required for flight under visual flight rules (VFR). When IMC exists, the pilot cannot legally or safely navigate by outside visual references and must fly by reference to cockpit instruments under instrument flight rules (IFR).
Plain English
Weather that is too poor to fly by looking outside. The pilot must rely on the cockpit instruments to know where the aircraft is, which way it is pointing, and whether it is level.
Context Anchor
Seen in instrument flying, weather briefings, departure planning, and descriptions of takeoff or climb conditions when clouds, low ceilings, or poor visibility affect the flight.
Derivation
From Latin instrumentum (a tool used for a task) and Greek meteoros (high in the air), the root of meteorology, the study of the atmosphere. Together the phrase means 'atmospheric conditions that require the pilot to use instruments,' which captures the operational reality precisely.
Why Pilots Care
Determines whether VFR flight is possible or IFR must be used, affecting flight planning, required equipment, and safety margins.
Grounding Statement
Picture flying into a thick cloud layer where you cannot see the ground, the horizon, or any other aircraft. Everything you need to fly the airplane safely now comes from the instruments on the panel in front of you. That situation is IMC.
Intuition Check
IMC does not always mean the airplane is inside a cloud. It means the weather is below the visual-flight minimums, so outside references are not enough for normal visual flying.
Example Sentence 1
The forecast showed IMC along the entire route, so the pilot filed an IFR flight plan before departure.
Example Sentence 2
The forecast called for instrument meteorological conditions along the route, so an IFR flight plan was filed.