right|thumb|250px|An automobile airbag, like this one in a crashed SEAT Ibiza car, deflates after 0.3 seconds.
An "airbag", also known as a Supplementary/Secondary Restraint System (SRS) or as an Air Cushion Restraint System (ACRS), is a flexible membrane or envelope, inflatable to contain air or some other gas. Air bags are most commonly used for cushioning, in particular after very rapid inflation in the case of an automobile collision.
In 1952 the airbag was invented by John W. Hetrick and he patented the airbag the following year. It was an invention to help protect his own family using expertise from his naval engineering days.
There have been airbag-like devices for aeroplanes as early as the 1940s, with the first patents filed in the 1950s.
The American inventor Allen Breed then developed a key component for automotive use - the ball-in-tube sensor for crash detection. He marketed this innovation first in 1967 to Chrysler. During this era, Americans were infrequent users of seat belts and a means of offering seat belt-like levels of occupant protection to unbelted occupants in a head-on collision was felt to be a valuable innovation.