What is referred to as the actual height above sea level?

The term that refers to the actual height above sea level is known as True Altitude. This measurement indicates the vertical distance of an aircraft above the mean sea level (MSL), which is a standardized reference point for altitude measurement. True altitude is critical in aviation as it provides pilots with accurate positioning in relation to the earth's surface, especially for navigation, terrain avoidance, and when flying in controlled airspace.

In contrast, pressure altitude is based on atmospheric pressure and does not necessarily correspond to a height above sea level. Flight level is a standardized altitude expressed in hundreds of feet, but it is based on a specific pressure setting, which may vary from the actual height above sea level. Relative altitude considers height above a specific reference point (such as ground level) rather than sea level. Hence, true altitude is the most precise measure when discussing an aircraft's elevation in relation to sea level.