A persistent world (PW) is a virtual world that continues to exist even after a user exits it, and to whose state user-made changes are to some extent permanent. The term is frequently used in the definition of massively multiplayer online video games and can be considered synonymous with that class of game, including other narrative forms of a media franchise.
'Persistence' comes from maintaining and developing a single or dynamic instance state of the game world that is shared and viewed by all players around the clock. Quite unlike other types of games, the plot and events in a single-permainstance-world game continue to develop even while some of the players are not playing their characters. This is similar to the real world in which events occur whether or not they are directly or indirectly related to a person, and (for instance) continue to happen while a person is asleep. Conversely, a player's character can also influence and change a persistent world to a degree that varies from game to game. Since the game does not pause or create player-accessible back-up files, a character's actions will have consequences that the player must deal with.
Elements of persistent worlds can be found in computer games from as early as the 1980s, including Trade Wars (1984), Wasteland (1988), and Orb Wars (1989). Several video games from the 1980s also featured elements of persistent worlds with their own day-night cycles and non-player characters going about their own daily routines, including Konami's Castlevania II: Simon's Quest (1987), Arsys Software's Reviver (1987), Nintendo's Zelda II: The Adventure of Link (1987), T&E Soft's Hydlide 3: The Space Memories (1987), Fun Factory's War of the Dead (1987), and Chunsoft's Dragon Quest III (1988). Certain aspects of persistent worlds became more and more prominent in the Pokemon franchise beginning with Pokemon Gold.
The term "persistent world" gained popularity in the late 1990s with the growth in popularity of MMORPGs. The term is also frequently used by players of Neverwinter Nights (2002) and Neverwinter Nights 2 (2006) to refer to MMORPG-like online environments created using the toolkits of games such as Arkaz, Avlis, Dasaria, and The Known Lands (TKL).
Offline persistence 
Persistent worlds can be simulated in offline games, as in the Animal Crossing series. Even though nothing happens while the game is off (due to the obvious technical constraint), the computer creates the illusion of persistence by advancing events as soon as the game is turned on. The game generates events that could have happened during the real-world time in which it was inactive. NPC development is also seen as a key factor in maintaining the impression of offline and online persistence. A method for creating NPCs who evolve and adapt to gameplay is seen in research done on "motivated reinforcement learning agents." Further persistence is possible if NPCs interact with players in an adaptive way rather than statically and without regard to prior experiences. Another good example of offline persistence is in Metal Gear Solid 3. If one stops playing long enough during the fight of "The Boss" with "The End", he will die of old age. Perhaps the first game to use this technique was the game Noctis, in which players are advised to turn off the game while refueling because it takes so long. In this game, if a player who has landed on a planet stops playing and then after a while resumes, he or she can see visible changes in the sea level or the daytime/nighttime cycle.
See also 
- Deal, David (2007). "Gamer Culture: A Brief History of Community and Digital Games". Retrieved 2008-04-06.[dead link]
- Marcellino, Bill. "Neverwinter Nights: Player's Guide to Persistent Worlds". Retrieved 2008-04-06.
- James, Daniel (Ed.); Gordon Walton (Ed.) (2004). "2004 Persistent Worlds Whitepaper". IGDA. Archived from the original on 2008-02-28. Retrieved 2008-04-06.
- Koster, Raph (2002-02-20). "Online World Timeline". Retrieved 2008-04-06.
- Bartle, Richard (2003). Designing Virtual Worlds. New Riders. p. 14. ISBN 0-13-101816-7.
- Mike Whalen, Giancarlo Varanini. "The History of Castlevania - Castlevania II: Simon's Quest". GameSpot. Retrieved 2008-08-01.
- "Reviver". Oh!FM. Archived from the original on 2 September 2012. Retrieved 2 September 2012.
- The Gamasutra Quantum Leap Awards: Storytelling (Page 3), Gamasutra, November 3, 2006
- Kurt Kalata & Robert Greene, Hydlide, Hardcore Gaming 101
- John Szczepaniak, War of the Dead, Hardcore Gaming 101, 15 January 2011
- Vestal, Andrew (1998-11-02). "The History of Console RPGs". GameSpot. Retrieved 2011-01-06.
- Merrick, Kathryn and Maher, Mary Lou (2006). "Motivated reinforcement learning for non-player characters in persistent computer game worlds". ACE '06: Proceedings of the 2006 ACM SIGCHI international conference on Advances in computer entertainment technology 266: 1–3. doi:10.1145/1178823.1178828.