Plain English Meaning

Weather bad enough that you can't see well enough outside the aircraft to fly visually, so you have to fly using only what the cockpit instruments are telling you.

Full Definition

Weather conditions expressed in terms of visibility, distance from clouds, and ceiling that are less than the minimums specified for visual flight rules (VFR), requiring the pilot to fly by sole reference to the aircraft instruments rather than outside visual cues.

Context Anchor

You will see IMC in weather briefings, flight planning, navigation discussions, and warnings about losing outside visual reference in .