(Sanskrit सुषुम्णा) A name for the semi-physical channel (nadi) of energy in the spinal column. In TIbetan Buddhism, it is called uma.
"When we study the construction, location and function of the Spinal Cord and the Sushumna Nadi, we can readily say that the Spinal Cord was called Sushumna Nadi by the Yogins of yore. The Western Anatomy deals with the gross form and functions of the Spinal Cord, while the Yogins of ancient times dealt with all about the subtle (Sukshma) nature. Now in Kundalini Yoga, you should have a thorough knowledge of this Nadi. Sushumna extends from the Muladhara Chakra (second vertebra of coccygeal region) to Brahmarandhra. The Western Anatomy admits that there is a central canal in the Spinal Cord, called Canalis Centralis and that the cord is made up of grey and white brain-matter. Spinal Cord is dropped or suspended in the hollow of the spinal column. In the same way, Sushumna is dropped within the spinal canal and has subtle sections." - Swami Sivananda, Kundalini Yoga
"The Hindus state that within the medullar canal there is a channel called Sushumna, and within this channel is another called Vajrini and within this a third called Citrini, which is "as fine as a spider's thread on which the chakras are threaded, like the knots on a bamboo pole." Thus, this is what the sacred books of India state, and we know that the Kundalini rises through the Citrini solely and exclusively with the Maithuna, Sexual Magic, Arcanum A.Z.F." - Samael Aun Weor, The Perfect Matrimony