A simulation is the operation of simulating, using a simulator or however experimenting a fictitious situation that can show practically the eventual real effects of the conditions that are used by the operator in his test.
Turing uses the term "simulation" to refer to what happens when a digital computer runs a state transition table (runs a program) that describes the state transitions, inputs and outputs of a subject descrete-state machine. The computer simulates the subject machine.
Simulation is often used in the training of civilian and military personnel. This usually occurs when it is prohibitively expensive or simply too dangerous to allow trainees to use the real equipment in the real world. In such situations they will spend time learning valuable lessons in a "safe" virtual environment.
Training simulations typically come in one of three categories: