Sanskrit नदी, "river, conduit, artery"; Tibetan tsa. Nerve channel for subtle energies.
“The term Nadi comes from the root Nad which means motion. The body is filled with an uncountable number of Nadis. If they were revealed to the eye, the body would present the appearance of a highly-complicated chart of ocean currents. Superficially the water seems one and the same. But examination shows that it is moving with varying degrees of force in all directions.” - Swami Sivananda, Kundalini Yoga