Kingpin

Suspension, showing tie rod, steering arm, king pin (axis) ball joints

The kingpin is the main pivot in the steering mechanism of a car or other vehicle. Originally this was literally a steel pin on which the moveable, steerable wheel was mounted to the suspension. In more modern designs, it may not physically exist as an actual pin, but the axis around which the steered wheels pivot is still referred to as the kingpin (or kingpin axis). It is usually made out of metal.