Plain English Meaning

The fast spinning of the small wheel inside a gyro instrument. That spinning is what lets the instrument hold a steady reference and show the pilot what the airplane is doing.

Full Definition

The spinning motion of a gyroscope's rotor about its spin axis, which gives the gyro its rigidity in space and makes it useful as a reference for flight instruments. In a turn coordinator, the rotor's spin axis is canted upward about 30 degrees from the longitudinal axis, so the gyro senses both roll and yaw rates rather than yaw alone.

Context Anchor

Seen in discussions of gyroscopic instruments, especially how the turn coordinator works.