Density altitude is defined as:

Density altitude is best understood as the altitude in the atmosphere at which the air density is equal to a specific standard condition. It is calculated based on the pressure altitude adjusted for non-standard temperature. The significance of density altitude lies in its impact on aircraft performance; as the air density decreases, which can occur with higher temperatures or elevations, aircraft require longer takeoff distances and may have reduced climb performance. By adjusting pressure altitude for temperature, density altitude provides a practical measure that helps pilots assess the performance capabilities of their aircraft in varying atmospheric conditions. This parameter is crucial for flight planning and safety.

The other definitions do not capture the specific nature of density altitude. For instance, altitude read directly from the altimeter represents current atmospheric conditions rather than density, height above the surface is a general term useful for measuring elevation but lacks precision in terms of air density effects, and actual height above mean sea level does not incorporate temperature effects, which are critical for determining density altitude.