Define the term 'altitude' in aviation.

The term 'altitude' in aviation refers specifically to the vertical distance of an aircraft above sea level or above the ground. This measurement is crucial for various reasons including navigation, safety, and compliance with air traffic control regulations. Altitude can be expressed in different ways, such as true altitude, which measures the distance above sea level, and absolute altitude, which is the height above the terrain directly beneath the aircraft.

Understanding altitude is fundamental for pilots as it affects aircraft performance, fuel efficiency, and air pressure. Additionally, knowing the altitude is essential for avoiding obstacles and maintaining safe separation from other aircraft.

The other options do not accurately represent the definition of altitude. One option relates to speed, which pertains to the horizontal motion of the aircraft, another concerns the weight at takeoff, which is relevant to performance and stability but not altitude, and the final option describes the rate of ascent or descent, which is about how quickly the aircraft gains or loses altitude rather than what altitude itself is.