|other||A Collider involved in this collision.|
OnTriggerEnter happens on the FixedUpdate function when two GameObjects collide. The Colliders involved are not always at the point of initial contact.
Note: Both GameObjects must contain a Collider component. One must have Collider.isTrigger enabled, and contain a Rigidbody. If both GameObjects have Collider.isTrigger enabled, no collision happens. The same applies when both GameObjects do not have a Rigidbody component.
For more information and code samples see API documentation on Collider.OnTriggerEnter.