A device used to help stabilize the yaw of a helicopter. They come in three forms right now. Mechanical, Piezoelectric, and Piezoelectric with heading hold. Mechanical gyros use a real spinning disk inside a small enclosure and help resist the yaw due to the torque of the main rotor blades by adjusting the tail rotor pitch. Piezoelectric gyros do the same thing, but are more accurate / responsive. See Heading Hold for the third type.