Caster angle is the angular displacement from the vertical axis of the suspension of a steered wheel in a car, bicycle or other vehicle, measured in the longitudinal direction. 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. Car racers sometimes adjust caster angle to optimize their car's handling characteristics in particular driving situations.
These were commonly known as a "pull to the left" cars. Manufacturers tend to build in preset alignment specifications and in many vehicles caster adjustment was simply not offered on an original vehicle suspension system.
When wheel-aligning a car, I always went to great lengths to correct the caster on a vehicle where possible, however this was not possible on W463 chassis ( g-wagen ) without caster bushing replacement.