1) A device used to measure or maintain orientation, for example, latitude, longitude, and altitude. It is based on the properties of angular momentum and usually consists of a spinning wheel or disk that is free to rotate to any orientation. 2) Often a ring laser is used as a gyroscope. Ring lasers consist of two beams of light of different frequency, the same polarization, and traveling in opposite directions around a closed loop. The loop often consists of a solid block of glass-ceramic material with holes drilled the length of all four sides, and mirrors attached to the corners to form a closed path. When the device is stationary, the optical paths traveled by the two beams are identical. Rotation of the cavity causes a difference in path length between the two beams that can be measured. 3) Fiber optic gyroscope (FOG): An instrument to measure angular rotation, based on the principle that the application of force will alter the wavelength of light as it travels in one direction or the other around a coil of optical fiber wound on a drum. The change in rotation causes a change in the interference pattern produced from the end of the coil, which can be measured.
1856, invented and named in Fr. 1852 by Foucault, from Gk. gyros "circle" + skopos "watcher," because the device demonstrates that the earth rotates.