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The angle of the steering axis when viewed from the side. More caster provides better straight-line stability and helps the steering self-center, but requires more effort to turn the wheel.
Seward's Race Car Design provides the precise definition: "Caster angle is the side-view angle between the centre-line of the wheel and the line passing between the upper and lower ball-joints (the steering axis). Caster trail is the longitudinal distance between these two lines at road level. Caster trail is the primary mechanism for providing the self-centring effect to the steering wheel. An element of caster trail is also required for straight-line stability." Jazar's Vehicle Dynamics Theory and Application adds that "when a castered wheel rotates about the steering axis, the wheel gains camber. This camber is generally favorable for cornering" — meaning caster provides dynamic negative camber gain when you steer, loading the outside tire's contact patch more evenly during cornering.