Persistent Shared World

State lives on the server and never resets. Characters, houses, items and prices persist between sessions, resources are shared rather than copied per player, and the consequences of other people's actions are waiting when you log in. Scale forces decisions single-player games never face: scarcity has to be managed, harassment has to be policed, and every mechanic is eventually exploited by someone with more time than the designers.

Origin Systems launched Ultima Online in 1997 with a simulated world of resources, crafting and player conflict. Credit here is contested and worth stating plainly: text-based multi-user dungeons had run persistent shared worlds since the late 1970s, and at least one graphical predecessor reached market before 1997. What Ultima Online demonstrated was that the model could work at commercial scale behind a subscription.

Persistence produced the most ambitious social experiments the medium has attempted, along with the discovery that a simulated world full of people behaves less like a story and more like a settlement, complete with theft, cartels and inflation. Costs are severe: servers, moderation and continuous new content, funded by subscriptions that require the world to keep growing, which is why so many closed and took their history with them.