•The goal: is the specific outcome that players will work to achieve
•The rules: place limitations on how players can achieve the goal.
•The feedback system: tells players how close they are to achieving the goal.
•The voluntary participation: requires that everyone who is playing the game knowingly and willingly accepts the goal, the rules, and the feedback.
Source: McGONIGAL, Jane. The reality is broken. London: The Penguin Press, 2011, pg. 21