Define "density altitude."

Density altitude refers specifically to the altitude at which the air density is equal to the standard atmospheric conditions at a temperature of 59°F (15°C) and a pressure of 29.92 inches of mercury (Hg), which represents a "standard day." This concept is fundamental in aviation because it affects aircraft performance, including engine efficiency, lift, and climb rates.

When flying, as temperature and atmospheric pressure change, actual air density can vary from standard conditions at a given altitude. Therefore, pilots calculate density altitude to assess how these variations impact aircraft performance. For example, on a hot day or at a high elevation airport, the air becomes less dense, which can significantly affect how well the aircraft can perform, highlighting the importance of understanding density altitude for safe flying.

The other options relate to navigation, altitude above sea level, and visibility requirements, which do not provide the specific scientific definition pertaining to air density that the correct answer encompasses.