A non-rigid airship, commonly called a blimp (/blɪmp/), is an airship (dirigible) without an internal structural framework or a keel. Unlike semi-rigid and rigid airships (e.g. Zeppelins), blimps rely on the pressure of their lifting gas (usually helium, rather than flammable hydrogen) and the strength of the envelope to maintain their shape. Blimps are known for their use in advertising, surveillance, and observation due to their maneuverability, slow speeds and steady flight capabilities.

Since blimps keep their shape with internal overpressure, typically the only solid parts are the passenger car (gondola) and the tail fins. A non-rigid airship that uses heated air instead of a light gas (such as helium) as a lifting medium is called a hot-air airship (sometimes there are battens near the bow, which assist with higher forces there from a mooring attachment or from the greater aerodynamic pressures there).

Volume changes of the lifting gas due to temperature changes or to changes of altitude are compensated for by pumping air into internal ballonets (air bags) to maintain the overpressure. Without sufficient overpressure, the blimp loses its ability to be steered and is slowed due to increased drag and distortion. The propeller air stream can be used to inflate the ballonets and so the hull. In some models, such as the Skyship 600, differential ballonet inflation can provide a measure of pitch trim control.

The engines driving the propellers are usually directly attached to the gondola, and in some models are partly steerable.

Blimps are the most commonly built airships because they are relatively easy to build and easy to transport once deflated. However, because of their unstable hull, their size is limited. A blimp with too long a hull may kink in the middle when the overpressure is insufficient or when maneuvered too fast (this has also happened with semi-rigid airships with weak keels). This led to the development of semi-rigids and rigid airships.

Modern blimps are launched somewhat heavier than air (overweight), in contrast to historic blimps. The missing lift is provided by lifting the nose and using engine power, or by angling the engine thrust. Some types also use steerable propellers or ducted fans. Operating in a state heavier than air avoids the need to dump ballast at lift-off and also avoids the need to lose costly helium lifting gas on landing (most of the Zeppelins achieved lift with very inexpensive hydrogen, which could be vented without concern to decrease altitude).

The origin of the word "blimp" has been the subject of some confusion. Lennart Ege notes two possible derivations:

Colloquially, non-rigid airships always were referred to as "blimps". Over the years, several explanations have been advanced about the origin of this word. The most common is that in the military vernacular, the Type B was referred to as "limp bag", which was simply abbreviated to "blimp". An alternative explanation is that on 5 December 1915, Commander A. D. Cunningham, R.N., of the Capel-Le-Ferne Air Ship Station, flicked the envelope of the airship SS.12 with his fingers during an inspection, which produced a sound that he mimicked and pronounced as "blimp", and that the word then caught on as the nickname for all small non-rigid airships.