An aerostat is a lighter than air craft including free balloons, airships, and moored balloons. Such a vehicle is lifted by buoyancy, containing a gas less dense than the ambient air within an envelope.
The term "aerostat" comes from the fact that buoyancy is technically said to provide "aerostatic" lift in that the force upwards arises without movement through the surrounding air mass. This contrasts with aerodynamic lift which requires the movement of at least some part of the aircraft through the surrounding air mass.
In technical usage, the term aerostat refers only to moored balloons. However, this article uses the term in its broader sense.