Defining virtual worlds - help!

I am working on a short definition of virtual worlds for an educational audience.  Here's what I have so far:

In a nutshell, virtual worlds are computer-generated, persistent, three-dimensional, multi-user spaces, where people interact with each other and the environment through their "avatars." Some examples of virtual worlds are Second Life, Whyville, World of Warcraft and Webkins. These three-dimensional environments often mimic characteristics of the real world, including having land, water, space, gravity, buildings and even weather.  Virtual worlds are populated by "avatars" which are controlled by human users, as well as non-player characters or "bots," which are controlled by software.  Different virtual worlds have different features and emphases, including the ability for users to communicate with each other using voice, to create objects and structures, to buy and sell goods and services, and to share video and audio content with each other.  Although often compared to video games, virtual worlds aren't necessarily games with explicit ways of "winning" or points to accumulate. 

A number of educational institutions are exploring the potential of virtual worlds as a new medium for education. Among the unique affordances of virtual worlds are: their capacity to facilitate engaging and immersive experiences, the ability for educators and students to be distributed anywhere in the world, the sense of "presence" and versimillitude, and the possibility of simulating phenomena and experiences that would be difficult or prohibitively expensive to do in the real world.

Did I miss anything important?  Is there anything incorrect or obtuse in this that bears unpacking?

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UPDATE 5/1/08: Rafi sent me Giff Constable's much more elegant and parsimonious definition of virtual worlds that totally pwns mine:

Virtual worlds are online environments that have game-like immersion and social media functionality without overarching game-like goals or rules. The core value is a sense of presence with others at the same time and in the same place.

Nice!

Comments

[Nexeus Fatale]

I feel your beginning is a bit, off, not by much how does this sound:

"In a nutshell, virtual worlds are computer-generated, persistent, three-dimensional, interactive online spaces, where people controlled representations of themselves, through avatars, are able to engage in a multi-user environment."

[Maxwell Biddle]

Rik: leave it as you have it.

The suggestions above don't enhance the definition and contain some basic grammar errors that would be glaring to an educators audience (or _should_ be!) as well as some sloppy concepts. One doesn't "engage in ... [an] environment" ... and your verb tenses don't agree. Further, "interactive" implies that the user _is_ "enagaged".

Sorry, Nexeus ... proofread before posting.

[rikomatic]

Thanks, guys. appreciated!

[The Riz]

Slight tweakin to knock it down a peg....

Virtual worlds are online environments that combine social communication with a video-game-like immersion, but without overarching game-like goals or rules. The core value is a sense of presence with others at the same time and in the same place.

Yes...I called them video-games. Who does that anymore?

[Shane]

I would say that virtual worlds aren't inherently multi-player, in that there are plenty of different video game 'worlds' that don't offer a multi-player experience. Virtual Worlds as a concept aren't necessarily synonymous with social platforms specifically.

[rikomatic]

I'm not sure that a digital environment that was experienced alone -- whether game-like or not -- would count as a virtual world in my book. I.e. is "Myst" a virtual world? Not in my opinion.