The bar that goes across a vehicle and holds two wheels, one at each end. The wheels turn around it (or turn with it — both designs exist). A car has two: a front axle and a rear axle. A big truck may have three or more, which is why toll roads and bridges sometimes charge by the number of axles.

It is a plain, everyday mechanical word — mechanics, drivers, and children with toy cars all use it. You hear it most when something has gone wrong: 'He hit the kerb and bent the axle,' 'a broken axle'. That is an expensive, stop-the-car kind of problem, so the word carries a little weight of trouble.

Be careful with axle and axis. They are cousins in origin but they do different jobs. An axle is a real metal bar you could hold. An axis is an imaginary line something spins around — the Earth's axis, or the x-axis on a graph. Nobody can touch the Earth's axis; a mechanic can touch your axle.