In aviation, what does the term "altitude" refer to?

The term "altitude" in aviation specifically refers to the height of an aircraft above sea level. It is a critical concept in aviation because maintaining appropriate altitude is essential for safe navigation, separation from terrain, and compliance with air traffic control regulations. The altitude is typically measured in feet or meters and can be indicated using several different references, such as pressure altitude or indicated altitude. Understanding altitude helps pilots determine their position within the airspace and ensures they can maintain safe vertical separation from other aircraft and obstacles.

The other choices pertain to different aviation concepts. The distance between aircraft relates to separation standards that ensure safety but does not define altitude. The pressure at flight level refers to how altitude is related to air pressure but is not the definition of altitude itself. Lastly, the speed of an aircraft relative to the ground is known as ground speed, which is distinct from the concept of altitude.