Plain English Meaning
A practice flight situation set up like a real instrument trip, complete with weather, clearances, and problems to solve, so the student learns to handle real instrument flying.
Full Definition
A training exercise built around a realistic flight situation conducted under Instrument Flight Rules, used by instructors to develop a pilot's ability to plan, fly, and make decisions in instrument meteorological conditions or in the IFR system. The scenario typically includes a route, weather, ATC interactions, possible failures, and decision points, and is flown either in an aircraft, a flight training device, or a simulator.