A region of the head that psychic information and dream stuff uses as a mechanism to alert conscious mind it is there it be perceived, It consists of the pineal gland.
There are two physical eyes, we all can see. There is not a third eye physiologically in your body, it is a metaphor. Two points must be understood; First, being energy of the third eye us really same as two physical eyes. The third eye is non-functioning, it cannot see unless physical eyes become unseeing. The same energy must move into third eye. It needs energy to function, and same energy has to be redirected.
Secondly, when you are seeing through physical eyes, you are seeing through physical body. Third eye is not part of physical body; but subtle body (suksham sharir). That's why physiology cannot believe that there is an existence of third eye.
With third eye functioning, you can enter a different dimension. With third eye you can see things which are invisible to the physical eye but visible to the subtle eye. With third eye functioning, you look at a person, you look at the person's soul, at the spirit, not at physical body through physical eyes, but you cannot see the soul.
The movement through third eye transforms into a new world, a subtle world. You start seeing things you have never seen.
The third eye is the conscience, the two physical eyes see the past and present, while the third eye reveals the insight of the future.