A steering geometry term; see diagram here. Caster is impacted the greatest when lifting a vehicle that has radius arms.

The angle which a kingpin makes with a vertical line, as viewed from the side. A positive caster angle tends to keep wheels in a straight line. Proper caster adjustments improve both tire wear and fuel.

Angle in side elevation of the forward or backward tilt of the steering axis. If the point of load is ahead of the point of contact, the caster angle is positive. The caster angle tends to keep wheels in a straight line and proper adjustment improves both tire wear and fuel economy.

The forward or backward tilt of the steering axis as viewed from the side. If the point of load is ahead of the point of contact, the caster angle is positive. The caster angle tends to keep wheels in a straight line. Proper caster adjustment improves both tire wear and fuel economy.

Caster (or castor) 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.