What is Consciousness?
By "consciousness", or "subjective consciousness", or any similar term in this essay, we do not mean any objective quality of a living creature that one could measure from the outside, with the obvious physical instruments. We do not mean, in particular, the ability of an organism to react in certain ways to stimuli, to behave so as to maximize the number of its descendants, or to maintain its sensitivity to its own situation. The question of how living things manage to have that kind of functional self-awareness is an interesting one, but it's not the question that we intend to address here.
(Note, on the other hand, that we're not simply assuming that no objective external measurements could ever tell us anything about the kind of consciousness that we do mean. It's possible that such measurements could eventually (by a process that we cannot at this point in our knowledge imagine) tell us something about consciousness. We are simply stressing the point that an explanation of how some organism's nervous system is wired in such a way that the organism is capable of certain behaviors is not an explanation of the kind of consciousness that we're concerned with here.)
What, then, do we mean by "consciousness"? The bibliography lists a number of works that treat this question in some detail. There seems to be no single unambiguous word for the concept that we want to pick out. Many words that can be used to refer to it (including "consciousness" itself) can also refer to objective, external, behavioral qualities of a system or organism; many others (such as "thoughts" or "feelings") refer only to parts of the whole. By "consciousness", we mean all of subjective experience, the inner narration of the mind, the "blooming, buzzing confusion" [William James, "The Principles of Psychology", 1890, chapter 13], the Cartesian theater (who coined that?); we mean what is it like to be a person (although as Nagel points out, we don't mean "similar to" by "like" here).
As in Frank Jackson's example, someone brought up in an entirely black-and-white environment could learn all the physical facts about the world, but still not know what it was like to see red; still not know the experience, the innerness, of the perception of a red thing. That aspect of redness, then, is part of the phenomenon of consciousness (as truths about the wavelength of light and the refractivity of materials are not).
I know that I am conscious, that I have subjective consciousness. I strongly suspect that you do also, and that by now you know pretty much exactly what I'm talking about; although of course one of the hard questions is exactly how I could come to know that about you. (See How can we know about any consciousness but our own?.) It would certainly be nice, though, if we had stronger and more definite terminology with which to isolate and discuss the problem (see A Skeptic about Consciousness for a thought experiment illustrating that aspect of the problem). We are also gathering personal anecdotes about consciousness, for whatever illumination they can contribute.