The caster (more properly “castor”) angle is the measurement of the difference between the suspension of a wheel and the steering axis.
Some caster angle is necessary to prevent wandering, making cars easier to drive with greater straight line stability.
Viewed from the side, caster angle is the longitudinal difference from vertical and the line between the upper and lower ball joints. Different displacements of the upper ball joint, changing the caster angle, are used to adjust the desired amount of tendency of tyres to follow in a straight line (“trailing”) or to be “damped” for greater control in cornering.
Caster angle is one of the key adjustments included in a full wheel alignment done by qualified technicians.