Air suspension is a type of vehicle suspension

Suspension (vehicle)

Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose — contributing to the car's roadholding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants...

 powered by an engine driven or electric air pump or compressor

Gas compressor

A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...

. This pump pressurizes the air, using compressed air as a spring. Air suspension replaces conventional steel springs. If the engine is left off for an extended period, the car will settle to the ground. The purpose of air suspension is to provide a smooth ride quality

Ride quality

Ride quality refers to the degree of protection offered vehicle occupants from uneven elements in the road surface, or the terrain if driving off-road. A car with very good ride quality is also a comfortable car to ride in. Cars which disturb vehicle occupants with major or minor road...

 and in some cases self-leveling.
Whilst not using high pressure mineral oil (as does the Citroën system), the system aims to achieve a result similar to the hydropneumatic suspension arrangement introduced in 1954 by Citroën

Citroën

Citroën is a major French automobile manufacturer, part of the PSA Peugeot Citroën group.Founded in 1919 by French industrialist André-Gustave Citroën , Citroën was the first mass-production car company outside the USA and pioneered the modern concept of creating a sales and services network that...

.

One of the earliest implementations of air suspension was by Messier

Messier

Messier can refer to:Surnames:*Charles Messier, French astronomer*Éric Messier, former NHL defenseman*Marc Messier, Canadian actor from Quebec*Mark Messier, NHL forward*Jean-Marie Messier, former CEO of Vivendi UniversalOther:*Messier object...

 in the 1920s.

With a "leg up" on other companies, GM used its experience with commercial buses' air suspension to introduce systems for its car lines, beginning with the 1958 model year.