A word, and all the infinite fluctuations it may possess. Like that moment when you know you have something to say, and you know you're speaking, even, but you still have no idea how you will say it. Or the moment when, as a reader, you're reading, and you are understanding what you are reading, but still have utterly no idea what will come next for you, what precisely the author wants to say. For me, that is the ultimate level of literary depth, of literary density.