"A copy for which there is no original." Examples would include the hacienda that Taco Bell stores are based on, Disneyland, MGM Studios, sex in pornography, and 40s "retro" fashions. The point isn't that there aren't "real life" models for these things--we could find a mission church similar to the Taco Bell store--but that the model becomes more important than any original we might discover. For example, in pornography you have highly scripted, often impossible sex that people then attempt to emulate in their lives. The representation becomes more important than the "thing itself." The signifier becomes more important than the signified. The term is central to the work of Baudrillard, althought Deleuze wrote about it much earlier.