Definition
True altitude is the actual vertical height of an aircraft above mean sea level (MSL), expressed in feet. It is the altitude corrected for nonstandard temperature and pressure variations from the indicated altitude shown on the altimeter.
Plain English
True altitude is how high you really are above sea level, not what the altimeter is showing. The altimeter can read a little high or low depending on the air's temperature and pressure, so the true altitude is the real number once those errors are taken out.
Context Anchor
Seen in Winds and Temperature Aloft Forecasts when identifying which forecast wind and temperature level applies to your planned altitude.
Derivation
‘True’ here means ‘actual’ or ‘real,’ as opposed to ‘indicated’ (what an instrument shows). Aviation uses several altitude types — indicated, pressure, density, true, absolute — and ‘true’ is the one that matches the aircraft’s real height above sea level.
Why Pilots Care
Accurate true altitude keeps the aircraft clear of terrain when temperatures or pressures deviate from standard.
Grounding Statement
In warmer-than-standard air your airplane is actually higher above the ground than the altimeter shows.
Intuition Check
Do not read true as meaning “better” or “more honest” here. In this context, true means measured from the fixed reference of mean sea level.
Example Sentence 1
On a cold day, the aircraft's true altitude was lower than the altitude indicated on the altimeter, so the pilot allowed extra margin over the ridge.
Example Sentence 2
Warmer air raised the airplane's true altitude above the indicated reading.