If you are making secret agreements with someone, then you are in collusion with them. Every time gas prices jump, consumers suspect the gas station owners of collusion and price fixing.
The noun collusion almost always implies that plans are being made because of some nefarious, most likely illegal, purpose. An art thief could be in collusion with a museum director to steal a famous painting from an art museum. Things get sticky when there is collusion between government and business, between union and management, or between your siblings.