A swingarm (originally known as a swing fork) is the main component of the rear suspension of most modern motorcycles and ATVs. It is used to hold the rear axle firmly while pivoting vertically to allow the suspension to absorb bumps in the road.
Originally there was no rear suspension - the frame design being a stronger version of a bicycle frame. Many types of suspension were tried including Indian's leaf spring suspended swingarm, and Matchless's cantilevered coiled spring swingarm. Immediately prior to and after WWII the "plunger" system in which the axle moved up and down two vertical posts became commonplace. In the latter, the movement in each direction was against coiled springs.
Some manufacturers such as Greeves used swingarm designs for the front forks which made them more robust then telescopic forks. The swingarm has also been used for the front suspension of scooters. In this case it aids in simplifying maintenance.