Interest curve for The Legend of Zelda: the Wind Waker; Bradley C. Buchanan
An interest curve is a graphical representation of a player's interest in a game over time.
In chapter 16 of his book The Art of Game Design: A Book of Lenses, game designer Jesse Schell explains the importance of keeping a player's interest by paying attention to pacing, and making sure the action escalates into a climax at the end of the game. Interest curves may also be drawn for individual levels or challenges within a game, and ought to follow the same escalating structure.