In the context of aviation and aerospace, an "Attitude Director Indicator" (ADI) is a type of flight instrument that provides the pilot with information on the aircraft's orientation and attitude relative to the ground and the horizon.

The ADI is an electro-mechanical instrument that combines the functions of a directional gyro (DG) and an attitude indicator (AI) into a single display. It shows the aircraft's pitch, roll, and yaw attitudes, as well as its heading and course deviation, all in a single, easy-to-read format.

The ADI typically consists of a large dial with a fixed horizon line and a movable symbol that represents the aircraft. The instrument uses a combination of gyroscopes and accelerometers to sense the aircraft's movements and display its attitude and heading information.

The main components of an ADI include:

1. Pitch scale: A vertical scale that indicates the aircraft's pitch attitude, with marks for various degrees of pitch (e.g., +10°, -10°).
2. Roll scale: A horizontal scale that indicates the aircraft's roll attitude, with marks for various degrees of roll (e.g., 10° left, 10° right).
3. Heading marker: A movable mark that indicates the aircraft's heading, relative to magnetic north.
4. Course deviation indicator: A pointer that indicates the aircraft's deviation from a pre-set course.

The ADI is used in conjunction with other flight instruments, such as the altimeter, airspeed indicator, and vertical speed indicator, to help the pilot control the aircraft's flight trajectory and maintain a stable attitude.

In modern aircraft, the ADI has largely been replaced by more advanced electronic flight instrument systems (EFIS), such as primary flight displays (PFDs) and navigation displays (NDs), which provide more comprehensive and intuitive information to the pilot. However, the ADI remains an important part of aviation history and is still used in some older aircraft and training simulators.