artificial horizon

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An instrument displaying a line on a flight indicator that lies within the horizontal plane and about which the pitching and banking movements of an aircraft are shown. Also called gyro horizon.

Wiktionary

1. n. aviation An instrument that displays the pitch and roll of an aircraft in flight by comparing its attitude to a gyroscopically-maintained indication of the flat horizon line.

GNU Webster's 1913

1. adj. See under Horizon
2. n. a level mirror, as the surface of mercury in a shallow vessel, or a plane reflector adjusted to the true level artificially; -- used chiefly with the sextant for observing the double altitude of a celestial body.

WordNet 3.0

1. n. a navigational instrument based on a gyroscope; it artificially provides a simulated horizon for the pilot