A type of reward system that gives rewards on an indeterminable and seemingly random schedule or ratio. You vary how often the person gets a reward when they do the target behavior. Opposite is the fixed-ratio reward system which rewards behaviors on a known schedule (ex. Buy 10 sandwiches get 1 free). The variable reward system is more effective and proven in psychological studies than the fixed ratio solution.
Slot machines are the easiest example showing the motivating and addictive effects of this type of system.
Author Nir Eyal suggests that there are 3 types of satisfactions that rewards can accomplish: tribal, hunt, and self.