simulation, simulacrum (2)
The Oxford English Dictionary defines simulation as the noun derived from the verb "simulate." It defines "simulate" as follows: 1. pretend to be, have, or feel. 2. imitate or counterfeit. 3. reproduce the conditions of (a situation etc.) e.g. for training. 4. produce a computer model of (a process)  Another word that belongs to this family is simulator.
The most commonly accepted use of simulation is in relation to an imitation of an apparatus or situation that has as its aim to bring about a certain behavior. Such simulations are used, for the most part, in instances of training such as in military operations. The idea behind this kind of simulation is to give an individual first hand experience of something he/she may later encounter in reality. This is a preparatory simulation as well as a testing device. These simulations are computer generated and are therefore able to alternate degree of difficulty or circumstances.
There exists another type of simulation having similar goals, but which is free from the pretext of training and instead offers an experience of an event or situation that the patron would otherwise not encounter. These experiences are also computer generated and are called simulators. Through them people can be exposed to a variety of events that range from trivial experiences such as a walk in a park, or those that are more exhilarating such as skydiving, without physically pursuing the activity.
As seen with the example of training for real experience, simulation rests on the existence of reality. Simulation is the resembling of something else, of a sign in the world. A photograph is also a resemblance of an already existing sign. However, a simulation differs from a photograph in that it exists in time. A simulation isn't a stilled image, but rather a kinetic experience. A related term that could be used to better explicate this is "to pretend." Because simulation relies on the existence of that which it is mimicking in reality, the simulation becomes unreal. Simulation rests on the traceability of its origins back to reality. That is to say that in order for a simulation to be named (i.e. "that is a simulation of...") that which it is attempting to represent has to be identifiable in the realm of the real. Because it is in opposition with reality, a simulation can be assumed to be false and have a negative connotation. It can be seen as deceitful with a malicious intent (i.e. cases involving false identities).
Because simulation is thought to be a false representation of something real, it is often thought that simulation is shallow. That is to say that in order for the simulation to be successful only the outer appearance of the "real" subject has to be imitated. A simulation therefore is a surface event. Furthermore this imitation need only last as long as the deceit is needed. In assuming a false representation, it is enough to mimic the outward appearance of the "thing", there is no need, nor is it possible to also mimic the essence of the subject. Of course it can be argued that taking on the outward appearance is the same as completely becoming something else. This idea is challenged in films such as Face/Off (John Woo, 1997). In this film the two main characters surgically change faces. The film attempts to show that the essence, or soul, of a person cannot be altered by outward appearance.
The Oxford Dictionary of Philosophy defines simulation as: "the view that our understanding of others is not gained by the tacit use of a 'theory', ...but by re-living the situation 'in their shoes' or from their point of view. Understanding others is achieved when we ourselves deliberate as they did."  Simulation was a topic for theoretical debate as early as in the writings of Plato and Aristotle. The Allegory of the Cave deals with the idea of simulation through the metaphor of puppets producing shadows on a cave wall. The simulation that Plato deals with exists on many levels. First there are the puppets that represent what is outside of the cave, and then there are the shadows on the cave wall, which are a simulation of the puppets. The puppets are a primary simulation whereas the shadows act as a type of second order simulation of the real world outside. Another term that gets used to talk about simulation, particularly in the writings of Aristotle, is mimesis. For Aristotle, mimesis is the representation of life, of reality. It is also natural part of life. Aristotle claims that humans have an innate propensity toward mimesis.  It is through mimesis that the real becomes apparent to us; it is how we learn about the real. It is not, as it is for Plato, a hindrance to our perception of reality. For Plato and Aristotle, simulation is directly linked to the real. That is to say that the real is identifiable (i.e. that which is outside the cave or that which is being represented). Certain contemporary theorists believe that due to multiple levels of simulation, the real is no longer identifiable. The existence of simulations as a representation of the real is therefore an argument rooted in history.
According to contemporary critical thought, the world has become composed solely of simulations. Theses simulations exist in layers and therefore cannot be traced back to their original subjects. In this way simulations have become based on other simulations, therefore the element of the real that they originally represent is unknowable. It is simulation that mediates reality. These multiple simulations give rise to the notion of simulacra. Simulacra refer to the layers of simulation present in the world.
The theorist that is largely responsible for these thoughts on simulation and simulacra is Jean Baudrillard. In noting that the representation, or simulation of signs was occurring in a layering effect, he created a system of the orders of simulacra. The orders of simulacra increase as it becomes less and less possible to trace the origins of the simulations. In effect the orders of simulacra function as a process whereby total simulacra is achieved. They are orders of simulation that progress until the difference between true and false has collapsed. That is to say that ultimately the simulacra is indistinguishable from the real. This is a historical process, in that it happens over time.
According to Baudrillard the world, as we know it now, is constructed on the representation of representations. These simulations exist to fool us into thinking that an identifiable reality exists. Baudrillard's orders of simulacra exist as follows: The first order of simulacra focuses on counterfeits and false images. In this instance the sign no longer refers to that which it is obligated to refer to, but rather to produced signifieds.  In this level, signs cease to have obligatory meanings. Instead the sign becomes more important than the physical. That is to say that the focus is placed on the sign rather than on what it is intended to represent. Thus what becomes crucial for the furthering of simulation is the reproduction of the sign itself not the physical or the signified. This is the realm of the automaton, the obvious fake that plays with reality.
The second order of simulacra is dominated by production of these false images. In this order signs become repetitive and begin to make individuals the same. Signs refer to the differentiation between the represented signifieds, not to reality.  This is the level of the robot, more real than the automaton, but not quite human. The robot con exist independent of human control in reality, but at the same time isn't real (see cybernetics).
The third order of simulacra rests on ultimate simulation. What is present in this order is the ultimate collapse between reality and the imaginary.  It is no longer possible to tell the difference between what is real and it's simulation. This is the level of the clone, not equivalent to man, but rather a hyperreal variant [see Reality/Hyperreality, (2)] . The clone is not man, but can pass undetected for man. In this instance it becomes impossible to produce simulations. For example you could not simulate a bank robbery in a real, functioning bank. You could not walk in to a bank with a fake gun, demand money and later claim, (after you have been arrested) that "It was only a simulation." The ramifications would be potentially equal to those of a real bank robbery. Simulation, as per the Oxford English Dictionary, is dead.
Because of this breakdown, it is through simulation that events are mediated. To this end Baudrillard takes as examples Disneyland, Watergate, and the Gulf War. We all know that Disneyland is not a "real" place, not a functioning city, but that it is an amusement park, an experience. For Baudrillard however this "false" place exists to mask the fact that the rest of America is not real.  He also claims that the Watergate scandal was a simulacrum to fool us into thinking that law and morality exist on a real level.  The Gulf War existed as a simulation to reinforce America's interests in the Middle East.  What these simulations do is mediate the truth. For Baudrillard simulation threatens the difference between true and false as well as between real and imaginary.  In these ways simulacra become that which conceals the truth and makes simulation, in the everyday sense, impossible.
What simulacrum does is engender myths of origin or reality.  Since we cannot trace simulations back to their source in reality, we are forced into a nostalgic area in which we attempt to remember what the real looks like. For Baudrillard such notions of concealment extend to sexuality, advertising and the media. The media controls "the mutation of events from the real into the hyperreal." 
Hyperreality is more real than real. It is "sheltered from the imaginary, and from the distinction between the real and the imaginary."  The hyperreal is that which is "always already reproduced."  The hyperreal is the world of simulation and simulacra; it is the reality we know because the real is truly unknowable. If we take the theoretical approach to simulation/simulacra we have to become aware that simulation in the everyday is no longer possible. Because the boundaries between the real and the imaginary are broken down, the simulation itself becomes impossible to identify.
In relation to the concept of mediation, simulation takes that very middle ground. That is to say that simulation as per Jean Baudrillard becomes the essence of mediation. It is not a medium, by which we receive messages, but rather it is a message in itself. In the tradition of Marshall McLuhan, the simulation becomes our knowledge of the real, not simply the intended representation of it. It is that which hinders our knowledge of the real and consequently our place in reality. This idea brings up interesting issues in relation to reality, does the real exist or has it been simulated so much that it is entirely impossible to recall what was once real? As a "media," Hollywood is a key factor in the production of simulation. It dictates our tastes and then reproduces only these preconceived notions of reality. Films attempt to depict reality, thereby dictating what reality should look like. In the end it becomes impossible to know what came first, the filmic depiction of reality or reality itself.
The media itself is therefore responsible for this breakdown of reality since it only provides us with simulated events and communications. As long as there is media, there will exist a simulation and reproduction of signs that constitute reality. The relation of media to simulation is an investigation into the idea of unknowable reality.