Plain English Meaning

It's when a pilot who was flying by looking outside suddenly flies into cloud or poor visibility they didn't plan for, and now has to fly using only the cockpit instruments.

Full Definition

An unplanned and unintended encounter with weather conditions that require flight by reference to instruments, occurring while the pilot is operating under visual flight rules. The pilot loses adequate outside visual references — typically due to cloud, fog, precipitation, or reduced visibility — and must transition immediately to instrument flying to maintain control of the aircraft.