Sage

A is someone who is able to utilise senjutsu and has mastered the art of Sage Mode. It is also a title given to someone because of their renowned power, wisdom, or holy standing.

Senjutsu
Senjutsu is taught by the toads of Mount Myōboku and the snakes of Ryūchi Cave. Whereas use of senjutsu is somewhat common amongst the animals that reside in these locations, very few humans have learned to perform senjutsu due to steep prerequisites and the inherent dangers of it.

Other Meanings

 * Hagoromo Ōtsutsuki, more commonly known as the Sage of Six Paths, was one of the first to be born with chakra, he saved the world from the Ten-Tails and created ninshū, a predecessor of the modern ninjutsu. He was capable of using the Six Paths Senjutsu which he, post-mortem, transfers as the Six Paths Sage Mode to his son's reincarnation.
 * Son Gokū has the moniker of.

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.
 * The Mount Myōboku, the Ryūchi Cave and the Shikkotsu Forest are called the, despite there being no apparent connection between Senjutsu and the slugs of the forest.