A plot event or change in game state that happens upon execution of a certain action, not because they are related, but because the author scripted it. This is a common method of keeping the plot in order.
A building that always exploded as soon as the PC left it, no matter how much time he spent there, would be an example. Leaving the building did not cause the building to explode, but it always happens at that time anyway. In "The Hitchkiker's Guide to the Galaxy", Ford Prefect doesn't show up until you have performed a certain action.