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 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 was one of the most famous sages in the series, a legendary figure who discovered chakra, created modern ninjutsu, the one who 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 current one, Obito Uchiha, is capable of doing. However, it is uncertain whether Hagoromo was able to use senjutsu before he sealed the beast within himself.
 * Son Gokū's moniker: The also makes mention of being a Sage.