Huyền Quan khiếu, also Huệ Quang Khiếu, Huyền khiếu, Tổ khiếu, Thượng đan điền, Thiên môn and Côn lôn đảnh, is an opening, through which a spirit leaves and enters the body, according to Caodaist Esoteric Practice.
The Eighth Female Buddha's Teachings on Esoteric Practice:
As mentioned earlier, a human head is inside a skull that protects a brain. The skull top is called mỏ ác (fontanel), beneath which is Nê Hườn Cung. Below the forehead is the eyes, between which is Huệ Quang Khiếu. Nê Hườn Cung and Huệ Quang Khiếu are invisible, so they cannot be studied by current anatomy.
Huệ Quang Khiếu is the third eye, which will be opened when you have converted Khí (mind) into Thần (soul). According to Buddhism, you have then obtained the six miraculous powers or the spiritual enlightenment when you reach this stage. This stage, however, is only the result of the second stage: converting Khí into Thần in Caodaist esotericism. You have to complete the last stage: converting Thần back to Hư - that is, your Chơn Thần (soul) travels everywhere in the universe.