Nāḍī (Sanskrit: नाड़ी, lit. 'tube, pipe, nerve, blood vessel, pulse') is a term for the channels through which, in traditional Indian medicine and spiritual theory, the energies such as prana of the physical body, the subtle body and the causal body are said to flow. Within this philosophical framework, the nadis are said to connect at special points of intensity, the chakras. All nadis are said to originate from one of two centres; the heart and the kanda, the latter being an egg-shaped bulb in the pelvic area, just below the navel. The three principal nadis run from the base of the spine to the head, and are the ida on the left, the sushumna in the centre, and the pingala on the right. Ultimately the goal is to unblock these nadis to bring liberation.

Nadi is an important concept in Hindu philosophy, mentioned and described in the sources, some as much as 3,000 years old. The number of nadis of the human body is claimed to be up to hundreds of thousands and even millions. The three principal nadis are ida, pingala, and sushumna. Ida (इडा, iḍā "comfort") lies to the left of the spine, whereas pingala (पिङ्गल, piṅgala "orange", "tawny", "golden", "solar") is to the right side of the spine, mirroring the ida. Sushumna (Suṣumṇa "very gracious", "kind") runs along the spinal cord in the center, through the seven chakras. The Shiva Samhita treatise on yoga states that, out of 350,000 nadis, 14 are particularly important, and among them, the three just mentioned are the three most vital.

While the sushumna came to be envisioned as a vertical channel extending upwards from the heart, navel region, or base of the torso, there is an old precedent for the idea that it extends, like the śaktitantu, to the feet; the Mataṅgapārameśvara, a comparatively early Siddhāntatantra, envisions the sushumna running from the tips of the big toes to the crown of the head via the navel and heart. This archaic model of a central channel extending to the feet, linking together the principal series of nine lotuses [i.e., Kapālīśabhairava, the four Devīs and the four Dūtīs] spanning the body’s axis from crest (śikhā) to feet (pāda), may underlie the Brahmayāmala’s conception of the śaktitantu.

The nadis play a role in yoga, as many yogic practices, including shatkarmas, mudras and pranayama, are intended to open and unblock the nadis. When the channels are unblocked by the action of yoga, the energy of kundalini uncoils and rises up the sushumna from the base of the spine. The ultimate aim of some yogic practices is to direct prana into the sushumna nadi specifically, enabling kundalini to rise, and thus bring about moksha, or liberation.

Several of the ancient Upanishads use the concept of nadis (channels). The nadi system is mentioned in the Chandogya Upanishad (8~6 cc. BCE), verse 8.6.6. and in verses 3.6–3.7 of the Prasna Upanishad (second half of the 1 millennium BCE). As stated in the last,

3.6 "In the heart verily is Jivātma. Here, a hundred and one nāḍis arise. For each of these nāḍis, there are one hundred nāḍikās. For each of these, there are thousands more. In these Vyâna moves."
3.7 "Through one of these, the Udâna leads us upward by virtue of good deeds to the good worlds, by sin to the sinful worlds, by both to the worlds of men indeed." (PU Q3)

The medieval Sat-Cakra-Nirupana (1520s), one of the later and more fully developed classical texts on nadis and chakras, refers to these three main nadis by the names Sasi, Mihira, and Susumna.

In the space outside the Meru, the right apart from the body placed on the left and the right, are the two nadis, Sasi and Mihira. The Nadi Susumna, whose substance is the threefold Gunas, is in the middle. She is the form of Moon, Sun, and Fire, even water also; Her body, a string of blooming Dhatura flowers, extends from the middle of the Kanda to the Head, and the Vajra inside Her extends, shining, from the Medhra to the Head.