Castor or Caster angle
Caster (or castor) angle is the angular displacement from the vertical axis of the suspension of a wheel in a car, measured in the longitudinal direction (angle of the kingpin when looked at from the side of the car).
It is the angle between the pivot line (in a car - an imaginary line that runs through the center of the upper ball joint to the center of the lower ball joint) and vertical. Caster angle can be adjusted to optimize a car's handling characteristics for particular driving situations.
The pivot points of the steering are angled in a way that a line drawn through them intersects the road surface slightly ahead of the contact point of the wheel. The purpose of this is to provide a degree of self-centering for the steering - the wheel casters around so as to trail behind the axis of steering. This makes a car easier to drive and improves its straight line stability (reducing its tendency to wander).
Excessive caster angle will make the steering heavier and less responsive, although, in off-road racing, large caster angles are used to improve camber gain in cornering.
