Plain English Meaning
Permission to fly in clouds and bad weather using only the cockpit instruments, given to pilots who hold a current instrument rating.
Full Definition
The legal authority granted to a pilot who holds a current and valid instrument rating to act as pilot in command of an aircraft operating under Instrument Flight Rules (IFR) or in weather conditions less than the minimums prescribed for visual flight.
Context Anchor
Seen when discussing instrument proficiency checks, which can restore a pilot’s authority to fly under instrument rules after the pilot no longer meets recent instrument-experience requirements.