blimp

n.

A small nonrigid airship used for observation or as a barrage balloon.

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, it relies on a higher pressure of the gas (usually helium) inside the envelope to remain buoyant.

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.