This is a theoretical concept mobilized by different scholars for very different purposes.  A common definition of simulacrum is a copy of a copy whose relation to the model has become so attenuated that it can no longer properly said to be a copy.  However, below are some of the dominant ways in which critical theorists have mobilized the concept of the simulacrum.

1.  Jean Baudrillard:  At least in the US, Jean Baudrillard’s definition of simularcrum has had more traction and resonance.  A theorist of postmodernism, Baudrillard thinks through the relationship between reality, symbols, and society.  He concludes that in high-capitalist societies “we breathe an ether of floating images that no longer bear a relation to any reality whatsoever”.  According to Baudrillard, simulation is the substitution of signs of the real for the real.  Simulacra are copies that depict things that either had no reality to begin with or that no longer have an original (think about Debord’s society of the spectacle taken ad infinitum).  He calls this condition as hyperreal, where sings no longer represent or refer to an external model.  In such a society human experience is a simulation of reality.  According to Baudrillard, when it comes to postmodern simulation and simulacra, “It is no longer a question of imitation, nor duplication, nor even parody. It is a question of substituting the signs of the real for the real.”  Baudrillard is not merely suggesting that postmodern culture is artificial, because the concept of artificiality still requires some sense of reality against which to recognize the artifice. His point, rather, is that we have lost all ability to make sense of the distinction between nature and artifice. To clarify his point, he argues that there are three “orders of simulacra”: 1) in the first order of simulacra, which he associates with the pre-modern period, the image is a clear counterfeit of the real; the image is recognized as just an illusion, a place marker for the real; 2) in the second order of simulacra, which Baudrillard associates with the industrial revolution of the nineteenth century, the distinctions between the image and the representation begin to break down because of mass production and the proliferation of copies. Such production misrepresents and masks an underlying reality by imitating it so well, thus threatening to replace it (e.g. in photography or ideology); however, there is still a belief that, through critique or effective political action, one can still access the hidden fact of the real; 3) in the third order of simulacra, which is associated with the postmodern age, we are confronted with a precession of simulacra; that is, the representation precedes and determines the real. There is no longer any distinction between reality and its representation; there is only the simulacrum.

2.  Gilles Deleuze is another scholar who has worked with the concept of the simulacra.  Unlike Baudrillard Deleuze engages directly with Plato’s understanding of the simulacrum and reverses some of these terms.  A common definition of the simulacrum is a copy of a copy whose relation to the model has become so attenuated that it can no longer properly be said to be a copy. It stands on its own as a copy without a model.  (In what follows we are borrowing liberally from Brian Massumi’s description)

Deleuze  takes a similar definition as his starting point, but emphasizes its inadequacy. For beyond a certain point, the distinction is no longer one of degree. The simulacrum is not so much a copy twice removed than a phenomenon which  undermines the very distinction between copy and model.  The terms copy and model bind us to  the world of representation and objective (re)production. A copy, no matter  how many times removed, authentic or fake, is defined by the presence or  absence of internal, essential relations of resemblance to a model. The simulacrum, on the other hand, bears only an external and deceptive resemblance to a putative model. The process of its production is entirely different from that of its supposed model; its resemblance to it is merely a surface effect, an illusion.  Pop Art is the example Deleuze uses for simulacra that have
successfully broken out of the copy mold: the multiplied, stylized images take
on a life of their own (think Andy Warhol’s Campbell Soup Cans or images of Jackie O). The aim here is not to become an equivalent of the “model” but to turn against it and its world in order to open a new space for the simulacrum’s own mad proliferation. The simulacrum affirms its own difference.