What does ALT stand for?

Altitude

Elevation as measured in relationship to the earth's surface to sea level. OR Height OR The vertical distance between the airplane and the ground. OR The height of an object, like an airplane, above sea level or above the earth's surface. OR Height, usually with respect to the terrain below (radar altitude, feet above closest dirt) or fixed earth reference (barometric altitude, feet above mean sea level); Symbols: h; Typical Units: ft; Dimensions: Length; or  angle of a celestial object above the horizon.