Bindu

The subtlest chakra is ajna, but beyond ajna there is bindu. Its not a chakra, it is from  a different layer of reality. its beyond the realm of the mind. Since it is beyond the mind it is very difficult to explain it with normal logic or speculation. Bindhu is one of the many drop of nectar coming from sahasrara. Bindhu is hiranyagarbha, the golden egg or womb of creation. Every object evolves into material existence throught the medium of bindu. The center of bindhu is the core of all thing, from the sun to an atom.

“within bindu is contained the evolutionary potential for all the myriad objects of the univers. Bindu contain the blue print for creation.every object , conscious or unconscious, is link to the underlying essence of consciousness through the intermediary of the bindu.” Swami satyanada saraswati

Meaning: to split, it's the point where oneness divide itself to produce duality
Location: actually, bindu is a point without dimension. Bindu also mean voidness, the shoonya. It is the gateway to shoonya. The state of no-thingness, pure consciousness. For practical purpose we put it on the back of the head.
Symbol: The bindu is symbolized by both, a full moon and the croissant (phase of the moon), meaning that the spiritual aspirant has some glimpses of sahasrara but not the total experience.