Sanskrit ब्राह्मणादि "channel of Brahma" from ब्रह्मन् Brahma, the highest form of divinity, and nadī नदी, "river, conduit, artery." A subtle conduit in the center of the spinal column, through which energy can flow. In most people, it is dormant.
Hindu Tantra Yoga describes multiple conduits in the spinal column. The most well-known is Sushumna, described as having the form of Fire (vahni-svarupa); within it is the vajrini-nadi in the form of the sun (surya-svarupa). Within that is the chitrini-nadi, also called Brahma-nadi, in the form of the moon (chandra-svarupa).
Brahmanadi is the channel that the Kundalini rises through.
"The Brahmanadi or “canalis centralis” within which the Kundalini ascends exists throughout the length of the spinal medulla... Each one of our seven bodies has its own spinal medulla and its Brahmanadi." - Samael Aun Weor, The Mysteries of the Fire: Kundalini Yoga
"Within the Sushumna Nadi there is a Nadi by name Vajra. Chitra Nadi, a minute canal, which is also called Brahmanadi, is within this Vajra Nadi. Kundalini, when awakened, passes through Chitra Nadi." - Swami Sivananda, Kundalini Yoga