Plain English Meaning
A spinning wheel inside a flight instrument that is kept spinning by an electric motor. Because the wheel resists changes in its spinning direction, the instrument can use it to detect how the aircraft is moving.
Full Definition
A gyroscope whose rotor is spun by an electric motor powered from the aircraft's electrical system, rather than by a stream of air from a vacuum or pressure pump. The spinning rotor maintains a fixed orientation in space, allowing the instrument it drives to sense changes in aircraft attitude or rate of turn.
Context Anchor
Seen in discussions of the turn coordinator and other cockpit instruments that use gyros for or turn information.