An axle is a central shaft for a rotating wheel or gear. In some cases the axle may be fixed in position with a bearing or bushing sitting inside the hole in the wheel or gear to allow the wheel or gear to rotate around the axle. In other cases the wheel or gear may be fixed to the axle, with bearings or bushings provided at the mounting points where the axle is supported.
Rear-wheel-drive automobiles and other vehicles usually have a "split rear axle". That is, each wheel is attached to a separate shaft, with a differential sitting between the two shafts and providing means to transfer power from the driveshaft to the two "half axles". The differential and split axle allow the two wheels to rotate at different speeds when the vehicle is turning.
