An emergent system is a system in which simple rules or units combine to lead to unpredictable consequences (Bogost 95). Conway's mathematical exercise The Game of Life is probably the most well known of these systems, though any non-linear experience or game falls under this distinction.
In the parlance of game studies, Jesper Juul compares emergent games with progressive games, in which the player completes actions to move towards the game's final goal (Bogost 121). Emergent games, according to Juul, yield more replayability, and thus have clear design advantages.
Jesse Schell offers a practical guide to creating emergent gameplay in his book The Art of Game Design: A Book of Lenses (141-142):
- Add more verbs -- more possible actions leads to a higher level of emergence.
- [Add] Verbs that can act on many objects -- this leads to even more combinations
- Goals that can be acheived in multiple ways -- if there was only one way to complete a goal, the game is not as fleshed out as it could be
- Many subjects -- this includes pieces that play by different rules, like a piece that moves twice as fast as others. Multiple subjects make for multiple strategic options.
- Side effects that change constraints -- situations in which one player's move limits or changes the possibilities for another player make for very dynamic systems.