Plain English Meaning

Weather that's too poor to fly by looking outside. The pilot has to fly using cockpit instruments because clouds, fog, or low visibility make it impossible to see the ground or horizon clearly.

Full Definition

Weather conditions in which visibility, distance from clouds, or cloud ceiling are below the minimums required for flight under Visual Flight Rules (VFR), requiring the pilot to operate the aircraft by reference to instruments rather than by outside visual cues. Flight in IMC requires an and an active in .