Sage

A is someone who is able to utilize senjutsu (仙術, Sage Techniques), and has mastered the art of Sage Mode.

Senjutsu
Very few people are shown to master senjutsu. Naruto Uzumaki and Jiraiya are the only two human beings seen using it, with Naruto the only one of the two to perfect it. 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 petrified shinobi, who failed to master the art of senjutsu.

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-Tailed Beast and was the first jinchūriki.