Any vehicle capable of atmospheric travel at higher than ground-effect level. Depending on the local environment and technology available, the form the aircraft takes can vary greatly. It might be lighter than air (dirigible) or heavier than air; propelled by microscopic cilia, actuators or aerofoils or by macroscopic propellers or turbines; dumb, smart, or sentient; subsonic or supersonic, or vary in other ways. By definition aircraft are unable to function in a vacuum, although there are amphibious and triphibious vehicles that can function underwater, on land, and in the air. Also most exoatmospheric shuttles and fighter-craft can function equally well in an atmosphere or in a vacuum.