What term defines the actual distance between an aircraft and the terrain over which it is flying?

The term that defines the actual distance between an aircraft and the terrain over which it is flying is known as absolute altitude. This measure reflects the aircraft's height above the ground level, indicating the vertical distance to the surface beneath. Absolute altitude is determined using radar or other methods that assess the height directly above the terrain, which can vary widely depending on the topography.

In contrast, relative altitude generally refers to the height of an aircraft in relation to a reference point, such as sea level, rather than directly measuring the distance to the ground. True altitude is the vertical distance of the aircraft above mean sea level, which may not accurately describe the gap between the aircraft and the terrain beneath. Pressure altitude is based on atmospheric pressure readings and is primarily used during flight planning and performance calculations, lacking the specificity of measuring height above ground level.