gyroscope

n. Rotating wheel whose axis is free to turn but maintains a fixed direction unless perturbed, esp. Used for stabilization or with the compass in an aircraft, ship, etc.

n. 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.