Sahasrara Chakra is the name given to the point at the top of the brain in Kundalini Yoga. Sahasra means a thousand. Arah is a spoke or the radium. Chakra is the wheel. Thus Sahasrara chakra literally means a circle with a thousand spokes. It is also called sahasradala (a lotus with a thousand petals).
This name is given to the brain in Yoga literature, or rather to the cerebrum, because of the many fissures and convolutions forming the cerebral cortex.
A detailed description of the thousand petalled lotus is found in Shatchakra Nirupana (verses 39 to 55), where it is said that it is located above the two-petalled lotus called ajna chakra with the fifty syllables of the Sanskrit alphabet from a to la appearing, each twenty times on the one thousand petals. This lotus is the seat of Shiva in the human body.
Below the lotus is the upper end of the Shankhini nadi. The lotus shines white and bright like the full moon. Inside the lotus is a triangle with a vacuum, which is the abode of Shiva.
From there a constant flow of elixir is secreted from the moon, from its seventeenth aspect called nirvana kala. At the center of the nirvana kala, there is a very fine passage having the width of one thousandth part of a hair, wherein resides the atma, called jiva or hamsa.
Through this passage the adept takes his kundalini, after crossing the six lotuses and three knots. There it meets Shiva along with the jiva. Thereby the adept enters into the Samadhi state, which is liberation par excellence. Then the kundalini is taken back by the passage of kula or the muladhara lotus. Then there is no rebirth for the adept.
Bibliography
The Serpent Power (1958) by Arthur Avalon, published by Ganesh and Co Chennai
Aspects of Indian Thought (1966) Gopinath Kaviraj published by University of Burdwan Press – Burdwan.
Encyclopedia of Hinduism Volume IX page 37 - IHRF