A non-rigid airship, or blimp, differs from a rigid airship (e.g. a Zeppelin) in that it does not have a rigid structure that holds the airbag in shape. Rather, these aircraft rely on a higher pressure of the gas (usually helium) inside the envelope. Although a blimp can be steered and thus qualifies as a dirigible, that term is more commonly used with rigid airships.
Blimps are also distinct from moored balloons. While often very similar in shape, moored balloons (sometimes called aerostats) are tethered to the ground in contrast to blimps which are free flying aircraft.
Because 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.
The term "blimp" is reportedly onomatopoeic, the sound the airship makes when one taps the envelope (balloon) with a finger. Although there is some disagreement among historians, credit for coining the term is usually given to Lt. A.D. Conningham of the British Royal Navy in 1915.
There is an often repeated, but false, alternative explanation for the term. The erroneous story is that at some time in the early 20th century, the United States military had two classes for airships: Type A-rigid and Type B-limp, hence "blimp". In fact,
"there was no American 'A-class' of airships as such—all military aircraft, heavier or lighter-than-air were designated with 'A' until the appearance of B-class airships in May 1917. There was an American B airship—but there seems to be no record of any official designation of non-rigids as 'limp'. Further, according to the Oxford Dictionary, the first appearance of the word in print was in 1916, in England, a year before the first B-class airship." ("Etymology of 'Blimp'" by Dr. A. D. Topping, AAHS Journal, Winter 1963.)
The perpetuation of this erroneous explanation is an example of false etymology.