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 and Jiraiya are the only two human beings who are seen using it. This is because both learning and using senjutsu is not safe; death may easily occur while learning to use it. There are many toad sculptures on Myōbokuzan - these 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 and created modern ninjutsu.