Sanskrit ब्रह्मरन्ध्र "Brahma's opening" from ब्रह्म Brahma "supreme self" and रन्ध्र randhra, "opening, hole, cavity."
"“Brahmarandhra” means the hole of Brahman. It is the dwelling house of the human soul. This is also known as “Dasamadvara,” the tenth opening or the tenth door. The hollow place in the crown of the head known as anterior fontanelle in the new-born child is the Brahmarandhra. This is between the two parietal and occipital bones. This portion is very soft in a babe. When the child grows, it gets obliterated by the growth of the bones of the head. Brahma created the physical body and entered (Pravishat) the body to give illumination inside through this Brahmarandhra. In some of the Upanishads, it is stated like that. This is the most important part. It is very suitable for Nirguna Dhyana (abstract meditation). When the Yogi separates himself from the physical body at the time of death, this Brahmarandhra bursts open and Prana comes out through this opening (Kapala Moksha). “A hundred and one are the nerves of the heart. Of them one (Sushumna) has gone out piercing the head; going up through it, one attains immortality” (Kathopanishad)." - Swami Sivananda, Kundalini Yoga