A dirigible, sometimes also called an airship or a zep, is a neutrally buoyant atmospheric vehicle that is distinguished from other aerostats such as balloons by the fact that it can be steered and propelled. The lift comes from chambers that have a lower density than the surrounding air. This may be achieved by using a gas mix with lower average molecular weight, gas at a higher temperature, or in the case of vacuum dirigibles by using reinforced chambers that are at extremely low pressure.
If a dirigible's shape is maintained solely by internal pressure of its gas cells then it is called a blimp; if its shape is maintained by a rigid skeleton with gas cells attached then it is called a zeppelin. Such extremes are rare, and most dirigibles are semi-rigid, with a shape that is due to both factors. Popular usage does not always respect these technical niceties. For instance vacuum dirigibles are rarely called zeppelins and the semi-rigid models that have been used on gas giant planets by Floater societies ever since the Interplanetary Age are popularly known as 'zeps' in most Anglic languages.
Dirigibles were invented on Old Earth in the late Industrial Age in the first century BT, but were soon superseded by aerodynamic craft such as airplanes and helicopters and remained only in niche roles. This was because a dirigible is necessarily bulky and cannot match other aircraft for speed. The other problem was that the cheapest lifting gas available at the time, hydrogen, is highly flammable in a standard Terragen atmosphere and led to some major disasters in the early history of the technology. The alternative, helium, is quite rare on a Terrestrial Class world such as Old Earth and was expensive once the small reserves of the gas that were tapped as a byproduct of fossil fuel extraction had been exhausted. However, in the Information Age dirigibles saw a revival as cargo vessels. This was because the costs of sentient but non-sapient ai pilots declined, the cost of fuel increased, and the cost and efficiency of solar cells that could be mounted on the upper surface of a dirigible for power improved.
Dirigibles saw a major renaissance when Venus and Solsys' gas giant planets were first colonized and exploited by bubblehab dwellers, since an atmosphere that is cold or anoxic, or both, makes heated hydrogen a very suitable lifting gas and since with simple nanotechnology the materials for a dirigible can be constructed from the gases of the atmosphere itself. Dirigibles have remained a feature life on planets or in the larger habitats such as Bishop rings or Banks orbitals ever since. Fixed-wing craft are still preferred for rapid transport, but dirigibles of one kind or another are ubiquitous as automated cargo carriers and are also popular as leisure craft. Atlantean airships and their picturesque fancloth 'sails' are a well-known example of the latter.
Most bubblehabs are motile and might technically be called large dirigibles, but most consider them separately since their primary purpose is habitation rather than transportation.