ब्रह्मरन्ध्र, brahmarandhra

"cavity of Brahman"

Brahmarandhra chakra, also called nirvana-chakra, the opening of Brahma, is located in the area of ​​the fontanel bone on the crown of the head. Sometimes brahmarandhra is called sahasrara-chakra. This chakra has a thousand-petalled lotus, there is situated Guru Yogi Gorakhnath ji, Anupama-shakti. Brahmarandhra is the place that connects one with the universe, therefore it is considered an opening where Jiva connects with the macrocosm by rise of inner fire — Kundalini-Shakti.

In Siddha-siddhanta paddhati, the method of meditation on ether, the tattva of this chakra, is described:

"The eighth is the brahmarandra, nirvana-chakra. One must meditate on the streams of smoke, thin as the tip of a needle. There is Jalandhara-pitha, granting liberation..."

Siddha-siddhanta paddhati. Part 2. Shloka 8.

Jalandhara-pitha is a holy place, not far from Jvalamayi-pith, where Jalandharnath practiced. According to legend, he was born from the flame of a sacrificial fire, this is likely why he is identified with smoke.

Also there is known brahmarandhra-adhara, which is at the level of akasha-chakra (same area, but above the nirvana-chakra):

«The remaining sixteenth is brahmarandhra, akasha-chakra. There, one should always meditate on the lotus-like feet of bessing Guru. [Then sadhaka] becomes complete (purna), like the space.»

Siddha-siddhanta paddhati. Part 2. Shloka 25.