Strictly speaking, fun may be defined as the enjoyment of pleasure, particularly in leisure activities. Fun is subjective and usually accompanies some sort of voluntary activity.
In A Theory of Fun for Game Design, Raph Koster defines fun as "the act of mastering a problem mentally," or the feedback the brain gives us when we are absorbing patterns for learning purposes" (Koster 90, 96). Additionally, it is contextual in that people voluntarily seek out fun in certain situations; for example, school is often not fun because people take it seriously (96). To Koster, "fun is about learning in a context where there is no pressure, and that is why games matter" (98).
Similarly, game designer Jesse Schell advocates for the importance of fun in games in Chapter 3 of The Art of Game Design: A Book of Lenses. His succinct definition for fun is "pleasure with surprises," something games can excel at if designed correctly (Schell 26). Like Koster, Schell encourages designers to "find the fun" in games (26).
While Koster and Schell seem to imply that fun ought to be the main prerogative of game design, others tend to disagree. Theorist Ian Bogost, in Chapter 8 of Unit Operations: An Approach to Videogame Criticism, believes that this reliance "forces [Koster] into a corner," and that fun should be only one of the benefits of games, rather than the main one (Bogost 118). Similarly, the team of the Penny Arcade show "Extra Credits" holds that the focus on "fun" is holding the medium back (Season 5, Episode 8).