The attitude an direction indicator (ADI), formerly known as the gyro horizon or artificial horizon, is a flight instrument that informs the pilot of the aircraft orientation relative to Earth's horizon, and gives an immediate indication of the smallest orientation change. The miniature aircraft and horizon bar mimic the relationship of the aircraft relative to the actual horizon. It is a primary instrument for flight in instrument meteorological conditions.

The heart of the AI is a gyroscope that spins at high speed, from either an electric motor. The gyro is mounted in a double gimbal, which allows the aircraft to pitch and roll as the gyro stays vertically upright. An self-erecting mechanism, actuated by gravity, counteracts any precession due to bearing friction. It may take a few minutes for the erecting mechanism to bring the gyros to a vertical upright position after the aircraft engine is first powered up.

Attitude indicators have mechanisms that keep the instrument level with respect to the direction of gravity The instrument may develop small errors, in pitch or bank during extended periods of acceleration, deceleration, turns, or due to the earth curving underneath the plane on long trips. To start with, they often have slightly more weight in the bottom, so that when the aircraft is resting on the ground they will hang level and therefore they will be level when started. But once they are started, that pendulous weight in the bottom will not pull them level if they are out of level, but instead its pull will cause the gyro to precess. In order to let the gyro very slowly orient itself to the direction of gravity while in operation.