Virtual, n. 
(1) Artificial. Not real but life like. Everything within a MUD may exist
within a virtual reality. This reality is defined by the universe rules. 
(2) Among MUD programmers most often used to describe methods of
representing objects within a game in away not dependent on the most
commonly used rules of the game which is to say in short that non-standard
methods are employed in the process of creating and representing the object
to the player. For instance if rooms normally require separate data files
or associated sub-programs, a virtual cluster of rooms might have only one
associated sub-program and data file. The end player will generally not
perceive any difference between the virtual and non-virtual objects
presented to her. 
(3) Way to conceptualize data.