Sage

A "Sage" (仙人, Sennin) is someone who is able to utilise senjutsu (仙術; Literally meaning "Sage Techniques"), 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
Senjutsu (仙術; Literally meaning "Sage Techniques") refers to a specialised field of techniques that allows the user to sense and then gather the natural energy (自然エネルギー, shizen enerugī) around a person. Senjutsu practitioners can then learn to draw the energy of nature inside of them blending it with their own chakra (created from spiritual and physical energy within the shinobi), adding a new dimension of power to the sage's chakra, resulting in the creation of "senjutsu chakra" (仙術チャクラ, senjutsu chakura). This chakra cannot be seen by anyone other than those who have been trained in senjutsu. This new chakra enables the user to enter an empowered state called Sage Mode, which can then drastically increase the strength of all ninjutsu, genjutsu, and taijutsu. A person who is able to use senjutsu is called a sage (仙人, sennin). Currently, there are two known ways to learn senjutsu. One of them is to study under toads from Mount Myōboku, as Shima, Fukasaku, Jiraiya and Naruto have done. Gamakichi and Gamabunta also seem to be familiar with it. Another way is to study under snakes from the Ryūchi Cave where Kabuto Yakushi learnt it. 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 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 Uses
"Sage" can also refer to an individual renowned for their great power, wisdom, or holy standing. One of the most famous sages in the series was the Sage of the Six Paths, a legendary figure who discovered chakra, created modern ninjutsu, the one who saved the world from the Ten-Tails and was the first jinchūriki. It is unknown if he too was capable of using natural energy.