Plain English Meaning

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.

Full 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.