Definition
Height expressed in units of distance above a reference plane, usually mean sea level (MSL) or ground level (AGL). In aviation, altitude is reported in feet and is qualified by the reference used: MSL altitude is measured from average sea level; AGL altitude is measured from the terrain directly below the aircraft; pressure altitude is the altitude indicated when the altimeter is set to 29.92 inches of mercury; density altitude is pressure altitude corrected for non-standard temperature.
Plain English
How high the aircraft is, measured from a stated starting point — usually sea level or the ground beneath it.
Context Anchor
Seen constantly in flight planning, cockpit instruments, air traffic control instructions, weather reports, charts, and airport information.
Derivation
From the Latin altitudo, meaning 'height' or 'depth,' from altus, 'high.' The aviation use keeps the everyday sense of 'how high,' but pins it down by always stating what it's measured from.
Why Pilots Care
Correct altitude awareness prevents terrain collisions, maintains separation from other traffic, and satisfies regulatory airspace requirements.
Intuition Check
Altitude does not simply mean “high in the air.” In aviation, altitude must be tied to a reference, such as sea level or the ground.
Example Sentence 1
Example Sentence 2
High density altitude reduced engine and wing performance, making it harder for the airplane to reach a safe cruising altitude.