artificial horizon

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

• noun An instrument that indicates the pitching and banking of an aircraft by displaying the movement of a representation of the aircraft in relation to a line representing the horizon.

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

• noun 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.

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

• noun a navigational instrument based on a gyroscope; it artificially provides a simulated horizon for the pilot