The distance between the colliders at the contact point.
Physics engine keeps track of the distances between all the potentially interpenetrating colliders and once the distance is less than the sum of the contact offsets in a given collider pair, contacts are immediately generated and OnCollisionEnter event is sent. That means that while the contact pair is still active, the distance between the colliders can be greater than zero, equal to zero, or even less than zero when the colliders are still apart, touching, or overlapping respectively.
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