An airbag, also known as an Air Cushion Restraint System (ACRS), or the Supplemental Inflatable Restraint (SIR) (reflecting the airbag system's intened role as a supplement to conventional restraints such as seat belts), is a flexible envelope commonly used for cushioning against hard interior objects (such as steering wheels), in particular for rapid inflation in the case of an automobile collision. It is a "passive" safety component, rather than an "active" component (that is: it reacts after an accident has happened, rather than helping to avoid an accident in the first instance, like ABS or ESP).