(computer science) the technique of representing the real world by a computer program; "a simulation should imitate the internal processes and not merely the results of the thing being simulated"
the act of imitating the behavior of some situation or some process by means of something suitably analogous (especially for the purpose of study or personnel training)
From The Free On-line Dictionary of Computing: (http://www.foldoc.org/, Editor Denis Howe)
Attempting to predict aspects of the behaviour of some system by creating an approximate (mathematical) model of it. This can be done by physical modelling, by writing a special-purpose computer program or using a more general simulation package, probably still aimed at a particular kind of simulation (e.g. structural engineering, fluid flow). Typical examples are aircraft flight simlators or electronic circuit simulators. A great many simulation languages exist, e.g. Simula.