Imaginary line drawn through the center of the upper and lower ball joints on a conventional steering vehicle is known as the

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The steering axis. This is the imaginary line that runs through the centers of the upper and lower ball joints in the steering knuckle and represents the axis about which the wheel pivots when you turn the steering wheel. It’s the reference line for how the wheel’s steering geometry behaves, including how caster and other alignment effects come into play. It’s different from camber, which is the wheel’s tilt in the vertical plane; toe, which is the front-back alignment of the wheels relative to the vehicle centerline; and caster, which is the tilt of that steering axis in the side view that influences steering feel and self-centering. In older setups this line was called the kingpin axis, but with modern ball-joint suspension it’s best described as the steering axis.