A concept from
computer game software. "World modelling" describes the designer's attempt to create an
internally consistent (though not necesarily
realistic) game world. This is not the same as
level design; typically a game that employs world modelling only has one level: the world itself. As such, world modelling entails not only object placement but also things like
seasons,
weather,
object interaction and the like. And of course, the
computer-controlled people in a consistent world should also act consistently, so some
artificial intelligence (or at the very least, strong
scripting) is required.
World modelling is currently a lost art that waits to be rediscovered. People decry the mindlessness of current 3D games, but the richness and depth of games like Ultima VII weren't possible moments after 2D game technology was developed, either. The truth is that modelling a world in 3D is much more difficult than modelling a world in 2D was, and since 3D gaming is so prevalent nowadays, this is why we've seen a decline in the detail of the worlds presented to us.
Even a superb designer like Warren Spector was unable to fight this trend. Mr. Spector has long desired to make a game based on an extremely detailed model of a small area; he calls it the "one city block" RPG. The original design for Deus Ex called for several of these city-block squares - one in New York, one in Hong Kong and one in Paris, with all three being incredibly detailed and giving the player as much freedom of action with as little linearity as possible.
As the design of the game progressed, it became clear that even in this day and age, the technology to model an entire city block as one contiguous unit in 3D space and present it to the player in a playable fashion just wasn't there, and so he was forced to resort to a mission-based structure instead. The game had to be redesigned, and the world modelling goal of "one city block" was put off for a later game...perhaps Deus Ex 2.
What's the difference? We'll take an example, again from Deus Ex. In Hong Kong, you start on the street level in Tonnochi Road and can take an elevator up to the top of the VersaLife Corporation building. Once you get to the top of the building, you can look out a window down onto Tonnochi Road. In a modelled world, that would be the same Tonnochi Road - the same 3D space - that you just came from. But in Deus Ex, which is mission-based, it's not; Versalife is a different "mission" from Tonnochi Road, and what you're seeing is a facsimile designed to make you think that's where you just were.
Why should we care? A modelled world "hangs together" better than a mission-based one does - it has to, because everything has to fit in one data space. Suspension of disbelief is fickle, and you can actively want to suspend disbelief and not be able too if what you're seeing gives you too many inconsistencies. A modelled world has a much better chance of letting you suspend disbelief, making it possible to actually enjoy the experience more.