Your eyes are open, but nothing is registering. Unseeing describes a person, or more often a person's eyes or stare, that is pointed at the world while the mind is somewhere else entirely — lost in thought, in shock, in grief, or simply exhausted. Someone staring unseeing out of a train window is looking straight at the fields and taking in none of them. This is a literary word: novels and careful writing use it, ordinary conversation does not. If you said it at a dinner table it would sound like you were reading aloud from a book. It is not a medical word for blindness, though writers sometimes stretch it that way; a blind person's eyes might be called unseeing in poetry, but the ordinary word for that is simply 'blind'. Note the shape it usually takes: 'her unseeing eyes', 'he sat unseeing', 'staring unseeing at the wall'.