Plain English Meaning
A fast-spinning wheel inside an instrument that holds its position in space, giving the instrument a steady reference to measure how the aircraft is moving.
Full Definition
A spinning rotor mounted so that its axis is free to tilt in one or more directions. Once spinning at high speed, the rotor resists changes to the direction of its spin axis and provides a stable reference that gyroscopic flight instruments use to sense aircraft attitude, heading, or rate of turn.
Context Anchor
Seen in discussions of gyroscopic flight instruments, especially the attitude indicator, heading indicator, and turn coordinator.