An air spring is a suspension component that utilizes air as the compressing/resisting medium. Unlike a coil spring resisting medium air is infinitely adjustable and has an inherent progressive characteristic due to the molecular characteristics of atmospheric gasses. The amount of air in a given chamber dictates its level of resistance. The spring rate and characteristics of an air spring are also defined by the area the air inhabits and is compressed in.
Also see Air Chamber and Spring Curve