An airship or dirigible is a type of aerostat or lighter-than-air aircraft that can be steered and propelled through the air using rudders and propellers or other thrust mechanisms. Unlike aerodynamic aircraft such as fixed-wing aircraft and helicopters, which produce lift by moving a wing through the air, aerostatic aircraft stay aloft by having a large "envelope" filled with a gas which is less dense than the surrounding atmosphere. The first lifting gas used was hydrogen, although this had well-known concerns over its flammability. Helium was rare in most parts of the world, but large amounts were discovered in the USA. This meant that this non-flammable gas was rarely used for airships outside of the USA. All modern airships, since the 1960s, use helium.
The main types of airship are non-rigid (or blimps), semi-rigid and rigid. Blimps are "pressure" airships where internal pressure, maintained by forcing air into an internal ballonet, is used to maintain both the shape of the airship and its structural integrity. Semi-rigid airships maintain the envelope shape by internal pressure, but have some form of internal support such as a fixed keel to which control and engine gondolas, stabilizers, and steering surfaces are mounted. Rigid airships have a structural framework which maintains the shape and carries all loads such as gondolas and engines. The framework contains numerous balloons, known as "gas cells" or "gasbags" which supply static lift without having to bear any structural loading. Rigid airships are often called Zeppelins, as the type was invented by Count Zeppelin and the vast majority of rigid airships built were manufactured by the firm he founded.