Absolute altitude is a term used in aviation to describe the height of an aircraft above the actual landscape or terrain over which it is flying. It is the vertical distance between the aircraft and the point on the earths surface directly below it. When an altimeter measures altitude, it measures the vertical elevation above a reference point, which could be the mean sea level, the surface of the earth, or some other point. Absolute altitude varies with not only the height of the plane but also the height of the surface because it uses the ground or surface underneath the aircraft as its reference point.

There are different types of altitude measurements in aviation, and it is important to distinguish between them to ensure the safety of the parties communicating to each other about their vertical location in the sky. The different types of altitude include:

• Indicated altitude: The reading on the altimeter that appears when it’s set to the local barometric pressure at mean sea level.
• True altitude: The actual height of the aircraft above mean sea level.
• Pressure altitude: The height above a typical datum plane, which is also the altitude calculated by the altimeter when it’s set to the standard pressure setting (29.92 inches of mercury).
• Density altitude: The pressure altitude corrected for unusual temperature and atmospheric conditions.
• Absolute altitude: The height of the aircraft above the actual landscape or terrain over which it is flying.

Absolute altitude is commonly expressed in "feet AGL" (above ground level), and it is used to find many obstacles and airspace classifications that exist in feet above the ground.