is a hinged suspension link between the chassis and the suspension upright or hub that carries the wheel. Many factors come into play when replacing them, such as how you drive it etc. Tony's Toys has most everything you need for customizing the suspension on your side by side.
The inboard (chassis) at the end of a-arm is attached by a single pivot, usually a rubber bushing. It can thus control the position of the outboard end in only a single degree of freedom, maintaining the radial distance from the inboard mount. Although not deliberately free to move, the single bushing does not control the arm from moving back and forth; this motion is constrained by a separate link or radius rod.
This is in contrast to the wishbone. Wishbones are triangular and have two widely spaced inboard bearings. These constrain the outboard end of the wishbone from moving back and forth, controlling two degrees of freedom, and without requiring additional links.
Most A-Arms form the lower link of a suspension. A few designs use them as the upper link, usually with a lower wishbone. The additional radius rod is then attached to the upper arm.