A simulation is a system that behaves (internally and externally) similar to some other original system and is implemented in a completely different way from the other system that it is simulating. The underlying mechanisms used to recreate the scenario may be the same or different from the original one.

It can also be said that a simulator is a device that imitates the operation of another device, as it basically models the underlying states of the target system to the best possible way it can simulate.