True altitude is best defined as:

True altitude is best defined as the actual altitude above sea level. This measurement is important for pilots as it provides a reference point that is consistent and universally accepted, allowing for accurate navigation and obstacle clearance. True altitude takes into account the Earth's surface and the varying elevations of terrain, making it crucial for ensuring that aircraft maintain a safe height above ground level, particularly in mountainous regions or areas with significant elevation variations.

Using true altitude is vital during flight planning and operations since it affects performance calculations, clearances, and approaches. While indicated altitude can vary due to atmospheric pressure changes and is what is read from the altimeter, and pressure altitude pertains to a standard reference of 29.92 inHg, true altitude offers a more absolute measure of how high an aircraft is flying in relation to sea level.