Definition
A grouping term for the five distinct ways altitude is measured and referenced in aviation: indicated altitude (read directly from the altimeter when set to the current local pressure), true altitude (actual height above mean sea level), absolute altitude (height above the terrain directly below the aircraft, also called height above ground level or AGL), pressure altitude (height above the standard datum plane of 29.92 inches of mercury), and density altitude (pressure altitude corrected for nonstandard temperature). Each type answers a different operational question and is used in a different context.
Plain English
Altitude is not just one number. There are five different ways pilots talk about how high the aircraft is, and each one means something specific. One tells you what the altimeter reads, one tells you your true height above the sea, one tells you how high you are above the ground right below you, and the last two are used mainly to figure out how the aircraft and engine will perform.
Context Anchor
Seen in altimeter, performance, weather, and flight planning discussions.
Derivation
Altitude comes from the Latin altus, meaning “high.” That helps because every type of altitude is about height, but aviation must always ask: height measured from what?
Why Pilots Care
Choosing the wrong type leads to incorrect performance numbers, terrain clearance errors, or traffic separation problems.
Grounding Statement
The same aircraft can have several different altitude values at the same moment because each value is measured from a different reference.
Intuition Check
Do not assume altitude is one fixed number. In aviation, the type of altitude tells you what that height is being compared against.
Example Sentence 1
The instructor reviewed the five types of altitude with the student before the cross-country, focusing on the difference between true altitude and density altitude.
Example Sentence 2
The controller asked for indicated altitude while the pilot simultaneously tracked pressure altitude for the assigned flight level.