An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle.

In the former case, bearings or bushings are provided at the mounting points where the axle is supported. In the latter case, a bearing or bushing sits inside a central hole in the wheel to allow the wheel or gear to rotate around the axle. Sometimes, especially on bicycles, the latter type axle is referred to as a spindle.

On cars and trucks, several senses of the word axle occur in casual usage, referring to the shaft itself, its housing, or simply any transverse pair of wheels. Strictly speaking, a shaft which rotates with the wheel, being either bolted or splined in fixed relation to it, is called an axle or axle shaft.

However, in looser usage, an entire assembly including the surrounding axle housing is also called an axle.

More Info: en.wikipedia.org