Consciousness means that a person is aware of something; consciousness is always of something and it is never of nothing. This statement is pivotal in understanding what consciousness is.
Awareness becomes consciousness when one identifies something as something. When one identifies a chair as a chair with a certain motive, assigning a certain meaning to it, one is said to be conscious of it. On the other hand a mere viewing of a chair as a part of a general background, not giving any meaning to it cannot be called consciousness.
Consciousness, if it does not have an object in the described manner, remains a potential. It is actualized when it becomes consciousness of something, some object, in the manner described above.
Some people say that to be conscious also means that one is aware of space and time. So according to this view those who are not in their proper wits actually are not aware of space and time. This does not mean that they are not in space and time, and objects do not appear to them in space and time. It simply means that they cannot focus on space and time the way normal people can make them their object of consciousness, through assigning a meaning and value to them.
People who lose consciousness due to a disease cannot focus anything; they cannot simply recognize anything as a particular object. So a person who has lost his consciousness cannot recognize people and objects. If you ask such a person, showing him an object, what is this, when he is awake and his eyes are open, that person will not recognize that object. And this sign that the person is not conscious.
When a person is about to die , the first thing that leaves him is his consciousness, and that person cannot recognize even the dearest of his relatives and persons.
So, we can say that consciousness means to be aware of something, either inside one’s mind or outside. To be conscious of something does not entail that the object of consciousness necessarily has an outward existence. One can make fantastic or imaginary things, his object of consciousness. So one can say that he is aware of an angel, for he is thinking about an angel. Similarly one can make a unicorn his object of consciousness. Such things do not have a real existence but, philosophically speaking, only an intentional inexistence. Anything that becomes the object of our awareness or consciousness, and yet does not have any existence, is said to have an intentional inexistence. Such existence has a meaning and one has certain motives behind making nonexistent things his objects of consciousness.
Aristotle has described intentional inexistence in the context of sense perception. When our see something, according to Aristotle, it acquires the shape of that thing. So, if we see a car, our eye acquires the shape or form of that car and becomes like it. This acquired form has no matter or material part in it; therefore it has no real existence. Therefore such form is called existing intentionally, and having intentional inexistence.