Plain English Meaning
Whatever supplies the energy that makes an instrument or system work — usually the engine, the electrical system, or air pressure from outside the aircraft.
Full Definition
The energy supply that drives an aircraft instrument or system. In light aircraft, the most common power sources are the engine-driven vacuum (or pressure) pump, the electrical system (battery and alternator/generator), and ram air or static air pressure from the pitot-static system. Different instruments rely on different power sources, which is why a single failure does not disable every instrument at once.