What defines a flight level in aviation?

In aviation, a flight level is specifically defined as a measured pressure level above a standard datum, which is the International Standard Atmosphere. Flight levels are expressed in hundreds of feet, with the reference pressure typically set at 1013.25 hPa (hectopascals) or 29.92 inHg (inches of mercury). This standardization allows aircraft flying at different altitudes to maintain safe vertical separation, regardless of variations in local atmospheric pressure due to weather changes or geographical differences.

This definition is crucial because it ensures pilots and air traffic control can communicate clearly about the aircraft's altitude, independent of the actual elevation of the terrain below or current atmospheric conditions. By maintaining a standard, it facilitates safe and efficient operation within airspace, especially at cruising altitudes where many aircraft operate.

The other options refer to different concepts in aviation. Elevation above sea level pertains to the height of a point in relation to sea level, which varies by geographic location. Distance above ground level measures how high an aircraft is flying above the ground beneath it, which can also change during flight. Altitude in feet is a broader term that can refer to height above sea level or ground level but does not specifically indicate that it is based on pressure measurements