Virtual World
A virtual world can resemble a real or fictional environment or a blend of the two. A virtual world is designed to allow its inhabitants to interact with each other via their avatars (see Avatars definition). On paper, the new generation of virtual worlds will have no limits (although available computing resources must be a limiting factor at some point).
They will feature some degree of decentralization, at least with regard to their economy, and users will be able to buy and sell items in a realistic world that feels entirely believable.