"flowing water, a river, or current; any tube or pipe, especially a tubular organ (as a vein or artery in the body)"In yoga, nadi refers to channels in the subtle body through which prana and kundalini flow, as well as to the normal tubes and vessels of the physical body.The principle nadi is Sushumna, the central nerve channel connecting the chakras, and through which Kundalini Shakti is coaxed to rise. Sushumna is said to reside in the very center of the spinal cord. Wrapped around it are two secondary nadis, ida and pingala (loosely associated with the parasympathetic and sympathetic nervous systems).In many pranayama techniques, ida and pingala represent the left and right nostrils, respectively. Ida is feminine, yin, lunar, and receptive. Pingala is masculine, yang, solar, and active. Recent studies have shown that left-nostril lunar breathing, called chandra bheda, helps activate the right (more intuitive or synthetical) cerebral hemisphere, while right-nostril solar breathing, called surya bheda, helps activate the left (more logical or analytical) cerebral hemisphere.There are thousands of nadis extending throughout the subtle body, just as there are thousands of nerves and vessels running throughout the physical body. Shiva Samhita says there are 350,000 nadis, of which it elaborates on only fourteen; of these fourteen, only ida, pingala and sushumna are considered vitally important to the practice of yoga. Hatha Yoga Pradipika and other sources claim 72,000 nadis. These numbers are symbolic and based on Indic numerology; their exact significance may have been understood differently by different ancient teachers. All agree on the importance of ida, pingala, and sushumna, with sushumna being paramount.

This definition is in context to Classic Yoga. See more contextual defintions for Nadi.