gyroscope

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A device consisting of a spinning mass, typically a disk or wheel, mounted on a base so that its axis can turn freely in one or more directions and thereby maintain its orientation regardless of any movement of the base.

from The Century Dictionary.

• noun An instrument consisting of a fly-wheel, the axis of which can turn freely in any direction, designed to illustrate the dynamics of rotating bodies.

from the GNU version of the Collaborative International Dictionary of English.

• noun A rotating wheel, mounted in a ring or rings, for illustrating the dynamics of rotating bodies, the composition of rotations, etc. It was devised by Professor W. R. Johnson, in 1832, by whom it was called the rotascope.
• noun A form of the above apparatus, invented by M. Foucault, mounted so delicately as to render visible the rotation of the earth, through the tendency of the rotating wheel to preserve a constant plane of rotation, independently of the earth's motion.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun an apparatus composed of a wheel which spins inside of a frame (gimbal) and causes the balancing of the frame in any direction or position. In the form of a gyroscopic stabilizer, used to help keep aircraft and ships steady.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun rotating mechanism in the form of a universally mounted spinning wheel that offers resistance to turns in any direction