A moored balloon is an inflated fabric structure, often shaped like an airship and usually filled with helium, that is restrained by a cable attached to the ground or a vehicle. Moored balloons differ from airships and free balloons in that it is not free-flying. Moored balloons are sometimes called aerostats. However, the term aerostat can also be used to refer to all lighter-than-air aircraft. In this broader sense, moored balloons are a type of aerostat.