An airship is a dirigible oft called lighter-than-air (LTA) or buoyant aircraft, sometimes referred to as a blimp. Airships get lift (buoyancy) from the atmosphere using a large aerostat as a floatation aid (like a child’s water wings), which displaces the air it’s in in the same way that a ship’s hull displaces water for buoyancy to carry the rest of its weight. Aerostats are filled with LTA gas to hold their form against atmospheric pressure (i.e. puff them up) enabling the displacement necessary, which otherwise would squash them flat – when the displacement (so buoyancy) enabled is lost.
LTA gas is used to minimise aerostat weight, where the gas acts as a structural component (like a pit prop) to prevent its collapse. The larger the volume of the aerostat (so displacement) the greater the buoyancy! Airships of old used hydrogen for the purpose. However, hydrogen is combustible when mixed with air (oxygen) so its use largely was abandoned and the airworthiness authorities ban it, favouring safer and non-combustible (inert) helium, which is heavier. Nonetheless, the logic of this is based on fear rather than good aircraft engineering to mitigate risks involved, which all airborne vehicles must do to enable safe flight, and where it’s a hypocritical attitude when fuel (also combustible) is permitted (although with numerous rules to ensure safe use).
Since the aerostat of airships enables flotation in the atmosphere (like the hull of submarines in water), the engines are only needed to move the airship between points against winds, provide power for systems and to enable minor unbalanced weight to be borne aerodynamically or by vertical thrust. Heavier-than-air (HTA) aircraft require powerful engines to get them aloft and then remain airborne as well as to propel them between locations plus power their systems. As a result, for a number of uses needing exceptionally long endurance while carrying large volume payloads, LTA aircraft are able to achieve a considerable reduction in operating costs compared with HTA aircraft for the same purpose.