The theory or belief that reality is a simulation to a degree that it is indistinguishable from "truereality" and where the inhabitants of the simulation are unaware that they are living in a simulation.
"What if we are living in a simulation, what if none of this is real?"
"What is real? How do you define 'real'? If you're talking about what you can feel, what you can smell, what you can taste and see, then 'real' is simply electrical signals interpreted by your brain." - Morpheus