A state of full involvement in an activity.
Flow is being in the zone, when your crushing obstacles and gettin’ it done! This is an important concept in game design. Creating flow states usually means more enjoyable games. Sure there are games that thrive on the opposite of this, by being extremely hard and forcing the player to reload saves many times: However, most players enjoy being in the zone. To achieve a flow state you have to balance challenge and progress. If it is too easy, the game becomes boring; too hard, and the progress feels too slow to maintain that good feeling.
Flow can also be used for evil. Mobile gaming has pioneered timers and paywalls that interrupt flow states. By placing these in the middle desirable gameplay, they can increase the number of gamers willing to pay to maintain uninterrupted progress. Don’t do that! It’s a nasty psychological trick.