Plain English Meaning
When a pilot who was flying by looking outside suddenly can't see the ground or horizon anymore and is forced to fly using only the cockpit instruments. It wasn't planned -- the weather closed in or the pilot flew into something they didn't expect.
Full Definition
An unplanned encounter with weather conditions in which the pilot can no longer maintain visual reference to the ground or horizon and must rely on the aircraft's instruments to control the airplane. This typically occurs when a pilot operating under visual flight rules unexpectedly enters cloud, fog, heavy precipitation, or reduced visibility that drops below visual flight .