Collusion refers to the use of game characters to lead the player through a game, and is a way of the designer exerting indirect control over the player's actions. It was coined by game designer Jesse Schell in chapter 16 of his book The Art of Game Design: A Book of Lenses.
Schell's example of collusion comes from his work on the virtual reality experience Pirates of the Caribbean: Battle for Buccaneer Gold (294-295). Early in development, the design team wanted to give players freedom, and so created multiple islands they could visit, each ripe with different experiences. However, because players were too occupied fighting ships in the beginning of the game, no one paid much attention to the islands. They solved this problem by having the ships retreat after a certain point, towards one of the islands at random. Players followed and were able to experience what the designers wanted them to experience.