Plain English Meaning

A pilot flying by looking outside the airplane suddenly finds themselves in cloud, fog, or weather so poor they can no longer see well enough to fly visually. They didn't plan to be there — they ended up there by accident — and now they have to fly by their instruments instead.

Full Definition

An unintended encounter in which a pilot operating under Visual Flight Rules (VFR) enters Instrument Meteorological Conditions (IMC) — weather conditions in which outside visual references are no longer adequate to control the airplane by sight alone. The pilot must then transition immediately to control by reference to the and take action to exit the or obtain assistance.