, the brain-in-a-vat theory
is a variant of solipsism
. It countenances the notion that you are actually just a disembodied brain in some sort of suspension, attached by wires to a supercomputer that provides electrical input that simulates the existence of a real world, and responds to the brain's output in an appropriate way - a sort of virtual reality
That we are brains in vats is never seriously contended, but rather offered as an argument for philosophical skepticism. The argument is, if you cannot be sure that you aren't in this situation, how can you have any certain knowledge at all? After all, it seems that if you really were in this situation, the vast majority of your beliefs would be false.
It is thus very similar to Descartes' argument (in Meditations on First Philosophy) that centred around the possibility that an evil demon was controlling his every experience.
The status of the human race in the movie, The Matrix, is a reference to the brain-in-a-vat theory, though in that case their entire bodies were preserved, rather than just their brains.
See also: Simulated reality