In certain YOGA systems, a nadi is a subtle bodily channel used to redirect PRANA (the life force found within the breath). The goals are to alter consciousness, strengthen the body, and help bring about the transformation needed for libera-tion from the cycle of births.
In HATHA YOGA, prana is forced through these channels to aid in raising the KUNDALINI at the base of the spine. The most commonly known nadi is the sushumna nadi, which travels up the spinal cord. The ida and pingala nadis are depicted as widely rippling waves that intersect at each CHAKRA (psychic center), exactly mirroring each other as they ascend around sushumna.
Further reading: Swami Sivananda Radha, Hatha Yoga, the Hidden Language: Symbols, Secrets and Metaphor (Boston: Shambala, 1989); Pancham Sinh, trans., The Hatha Yoga Pradipika (New Delhi: Munshiram Mano-harlal, 1975).
Encyclopedia of Hinduism. A. Jones and James D. Ryan. 2007.