Altitude is the height from a given reference point or data plane.
In aviation, there are several different types of altitude. The primary references to altitude include: indicated altitude, true altitude, absolute altitude, and pressure altitude.
• Indicated altitude is the altitude shown on the altimeter.
• True altitude is the height above mean sea level (MSL).
• Absolute altitude is the height above ground level (AGL).
• Pressure altitude is the indicated altitude when an altimeter is set to 29.92 inches of mercury (Hg).
Density altitude is a related term but is not a reference to a specific distance from a data plane. Density altitude is pressure altitude corrected for nonstandard temperature variations. Unlike other altitudes, density altitude is a reference to performance, not actual height.
FAA AC-00-6B Aviation Weather pg. 5-13, 5-15
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge pg. 4-3
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge pg. 8-6, 12-5