Sage

A is someone who is able to utilise, and has mastered the art of Sage Mode or someone given said title because of their renown for their great power, wisdom, or holy standing.

Senjutsu
Very few people are shown to have mastered senjutsu. This is because both learning and using senjutsu is not safe; death may easily occur while learning to use it. Fukasaku claims that the toad sculptures on Mount Myōboku are those who failed to master the art of senjutsu. Naruto Uzumaki, Kabuto Yakushi, Hashirama Senju, Orochimaru, Minato Namikaze and Jiraiya are currently the only known human users of senjutsu. Other animals such as Fukasaku and Shima are also able to use it with the White Snake Sage and Great Toad Sage amongst others, being at least knowledgeable in it.

Other Meanings
"Sage" can also refer to an individual renowned for their great power, wisdom, or holy standing.
 * Hagoromo Ōtsutsuki, more commonly known as the, a legendary figure who was one of the first to be born with chakra, created modern ninjutsu, saved the world from the Shinju and was the first jinchūriki. As the first jinchūriki of the Shinju, he was able to use natural energy in his techniques, just as the latest ones, Obito and Madara Uchiha, are capable of doing. However, it is uncertain whether Hagoromo was able to use senjutsu before he sealed the beast within himself. He was capable of using the Six Paths Sage Technique which he, post-mortem, transferred to his son's reincarnation Naruto Uzumaki.
 * Son Gokū's moniker: The also makes mention of being a Sage.

Trivia

 * The term, refers to the Japanese term of Chinese sages and practitioners of Taoist ways, who have gained immortality and incredible powers through the manipulation of qi, or natural energy through their body by becoming ascetics and living on mountains, where qi is said to gather at the most and to start flowing from into the world. The training and measures needed to become a Sage in Naruto is just as vigorous and demanding, and similar in some regards, requiring one to have massive reserves of chakra, and come to adopt natural energy, not for the pursuit of immortality, but to become harmonised with the flow of the world around them.

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