Caster is the angle at which the steering axis of a wheel is tilted away from the vertical. It is measured in degrees and can be either positive or negative. Positive caster means that the steering axis is tilted back towards the rear of the vehicle, while negative caster means that the steering axis is tilted forward towards the front of the vehicle.
Caster is important for a number of reasons. It helps to stabilize the vehicle during cornering, reduces tire wear, and improves steering feel. The optimal caster angle for a particular vehicle will vary depending on the make, model, and suspension design.