Reincarnation (転生, Tensei, Viz: Transmigrant) is the process by which chakra of a deceased individual is reborn into a new living vessel. It is also a term that refers to a person that is a result of this process.
After the deaths of Indra Ōtsutsuki and Asura Ōtsutsuki, the two sons of the Sage of the Six Paths would continue to be reincarnated into their descendants centuries afterwards. Eventually, these chakras would be reborn into Madara Uchiha and Hashirama Senju, who continued the brothers' struggle until their own deaths. Prior to his death, Madara, thanks to the machinations of Black Zetsu, became the first reincarnation to successfully mix the chakra of Asura and Indra, which led to him awakening the Rinnegan.
After Madara's death, the chakra of Asura and Indra continued the cycle of reincarnation, being reborn into Naruto Uzumaki — a descendant of Asura through the Uzumaki clan — and Sasuke Uchiha of the Uchiha clan. As the first reincarnations to have been born into an era of relative peace, the two maintained a rather civil rivalry until Sasuke fell victim to his clan's Curse of Hatred.
The reincarnation cycle of Indra and Asura will not start again until both of their current reincarnations have passed away; the brothers did not reincarnate until Madara's death, which occurred decades after Hashirama's.
The chakras of Indra and Asura change chakra signatures with each reincarnation: although they are both Asura's reincarnations, Madara, a Sensor Type, was able to differentiate the chakras of Naruto and Hashirama.
- Through the Summoning: Impure World Reincarnation technique, both "reincarnation" and "resurrection" can be found; the sacrificed body is reincarnated into the body of the summoned shinobi, while the reincarnated shinobi is resurrected from the dead.
- Reincarnation Ninjutsu are techniques that usually require or accomplish the transfer of life force between people.