A philosophical zombie or p-zombie is a hypothetical entity that looks and behaves exactly like a human (often stipulated to be atom-by-atom identical to a human) but is not actually conscious. A p-zombie is as likely as anyone else to ask, "When I see red, do I see the same color that you see when you see red?" but they have no real experience of the color red; the zombie's speech can be explained in some other terms which do not require them to have real experiences. The zombie thought experiment is purported to show that consciousness cannot be reduced to merely physical things: our universe is purported to perhaps have special "bridging laws" which evoke a mind into existence when there are atoms in a suitable brain-like configuration.
Critics deny the possibility of zombies: if a p-zombie is atom-by-atom identical to a human being in our universe, then our speech can be explained by the same mechanisms as the zombie's, and yet it would seem awfully peculiar that our words and actions would have one entirely materialistic explanation, but also, furthermore, our universe happens to contain exactly the right bridging law such that our experiences are meaningful and our consciousness syncs up with what our merely physical bodies do. It's too much of a stretch: Occam's razor dictates that we favor a monistic universe with one uniform set of laws.