Gyroscope

[jahy-ruh-skohp] /ˈdʒaɪ rəˌskoʊp/

noun
1.
an apparatus consisting of a rotating wheel so mounted that its axis can turn freely in certain or all directions, and capable of maintaining the same absolute direction in space in spite of movements of the mountings and surrounding parts: used to maintain equilibrium, determine direction, etc.
/ˈdʒaɪrəˌskəʊp/
noun
1.
a device containing a disc rotating on an axis that can turn freely in any direction so that the disc resists the action of an applied couple and tends to maintain the same orientation in space irrespective of the movement of the surrounding structure Sometimes shortened to gyro
n.

heavy rotating wheel with an axis free to turn in any direction, 1856, invented and named in French 1852 by Foucault, from Greek gyros “circle” (see gyre) + skopos “watcher” (see scope (n.1)), because the device demonstrates that the earth rotates.
gyroscope
(jī’rə-skōp’)
An instrument consisting of a heavy disk or wheel spun rapidly about an axis like a top. The angular momentum of the disk causes it to resist changes in the direction of its axis of rotation, due to the principle of conservation of angular momentum. Because of the gyroscope’s tendency to remain oriented in one direction, it is used as a stabilizing device in missiles, as well as in the navigation and piloting systems of airplanes, ships, rockets, and other vehicles.