What do flight levels denote in aviation?

Flight levels in aviation specifically indicate an aircraft's altitude when it is flying at or above a certain standard pressure setting. This is typically measured in hundreds of feet, where each flight level represents an altitude at which pilots can safely fly without concerns regarding terrain. For example, Flight Level 350 refers to an altitude of 35,000 feet.

Using flight levels allows for standardization in air traffic control, ensuring that aircraft maintain safe vertical separation, particularly in controlled airspace. The adherence to a standard pressure setting (29.92 inches of mercury or 1013.25 hPa) when flying at these levels aids in consistency and safety, as it removes variability that could arise from changing weather conditions affecting barometric pressure.

On the other hand, the other options relate to different metrics that are not applicable to flight levels. Speed measured in knots pertains to how fast an aircraft is traveling, weight measured in kilograms relates to the mass of the aircraft or its cargo, and distance measured in nautical miles concerns how far an aircraft has moved or needs to move. Each of these metrics serves distinct purposes in aviation, but they do not define the concept of flight levels.