轴是轮子或齿轮中央的直杆或曲杆，大部份都为圆柱状，用于传动及支撑其他旋转组件。用于支撑的轴会被轮子或齿轮轴心洞内的轴承或轴套固定，这样，使轮子或齿轮能绕轴旋转；而用于传动的轴会把轮子或齿轮固定于轴上，而轴承或轴套则固定在其他结构上，用来支撑轴。 轴的分类：心轴，转动轴，传动轴，其中转动轴既受转矩又受弯矩，传动轴只受转矩，心轴只受弯矩。

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.