1. A lighter-than-air aircraft which sustains its height by the buoyancy of the surrounding air: Unpowered balloons, blimps, and dirigibles are examples of aerostats which use one or more containers filled with a lifting gas which keep them floating.
2. Any of a proposed system of satellites for use in air traffic control and maritime navigation: Observation aerostats are used by the military to watch the field of battle.
3. Etymology: from aero, [nautical] + sat [ellite].