Definition
In aviation, the height of an aircraft or object measured vertically from a defined reference. Several distinct types are used in flying: indicated altitude (read directly from the altimeter when set to the local pressure setting), true altitude (actual height above mean sea level), absolute altitude (height above the ground directly below), pressure altitude (height above the standard pressure datum of 29.92 inches of mercury), and density altitude (pressure altitude corrected for nonstandard temperature).
Plain English
How high something is, measured from an agreed starting point. In flying, that starting point changes depending on which kind of altitude you mean — sea level, the ground below, or a standard pressure setting.
Context Anchor
Seen in performance planning, aircraft operating discussions, weather reports, charts, and cockpit instrument readings.
Derivation
From the Latin altitudo, meaning height or depth. The plural form is used in aviation because pilots routinely work with several different kinds of altitude at the same time, each measured from a different reference.
Why Pilots Care
Each type of altitude tells the pilot something different — terrain clearance, aircraft performance, traffic separation, or fuel planning. Confusing one for another can lead to serious errors in navigation, performance prediction, and obstacle avoidance.
Intuition Check
Do not read altitudes as just a general idea of being high. In aviation, altitudes are measured heights tied to a specific reference point.
Example Sentence 1
The instructor explained the five altitudes a pilot must understand before flying cross-country.
Example Sentence 2
The airplane climbed through several altitudes while the controller assigned a new cruising level.