Pleasure with surprises. The essence of fun is surprise. Many game mechanics center around surprise, including turning cards over, looting corpses, and dice rolls. Challenge, difficulty, and uncertainty can add to surprise. Fiero is a feeling that can come from a surprising success.
(The Art of Game Design: A Book of Lenses by Jesse Schell, Carnegie Mellon University, 2008. Page 26; Slay the Dragon: Writing Great Video Games by Robert Denton Bryant & Keith Giglio, 2015. Page 145-146)
Game Design: The Lens of Fun
Fun is desirable in nearly every game, although sometimes fun defies analysis. To maximize your game's fun:
(The Art of Game Design: A Book of Lenses by Jesse Schell, Carnegie Mellon University, 2008. Page 27)