Trigger volume
A collider that only reports overlaps and fires events, without stopping anything that passes through it.
Where an ordinary collider pushes apart whatever touches it, a trigger volume lets objects pass straight through: the engine notices something enter or leave and sends a message, but resolves no contact. This is how you build invisible sensing zones — checkpoints, doors that open as the player nears, traps, sound cues, regions where enemies wake up.
In Unity it is a tick in the Is Trigger box with callbacks like OnTriggerEnter/OnTriggerExit; in Unreal they are trigger volumes raising overlap events. Because no impulse is computed they are cheaper than solid collisions, but they still take part in the broad phase. A common trap: two triggers sweeping past each other quickly can miss entirely without continuous collision detection.