What is the term used to describe aircraft that are in the air?

The term used to describe aircraft that are in the air is "airborne." This term specifically refers to any aircraft that has taken off from the ground and is currently flying. When an aircraft is airborne, it implies that it is actively operating within the atmosphere, utilizing lift generated by its wings or other mechanisms, and is not in contact with the ground.

In contrast, other terms provided describe different states: "grounded" indicates that an aircraft is on the ground and not in operation; "stationary" typically refers to something that is not moving, which can apply to aircraft on the ground; and "landborne" is not a commonly used term in aviation. Therefore, "airborne" is the most accurate term for describing aircraft that are currently flying.