Sage

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

Senjutsu
Very few people have been shown to be able to master the art of senjutsu. Since the energy of nature of energy is emmense, only those who already have enormous chakra capacities will have the capacity to go on and draw upon it. If the practitioner can achieve a perfect balance between the physical and spiritual energies of their own chakra, and the energies of nature, combining it with their own chakra, produces a new and stronger chakra, called "Sage Chakra": In order to explain this to Naruto in a way that he could understand it, Gamakichi likens the process of adding natural energy to one own physical and spiritual energies to adding an extra flavor to a Chocolate and Vanilla Swirl Ice Cream.

Those able to do so, be they Toad or Human, are able to enter a physical state, called "Sage Mode", the physical activation of ones own body in many ways, such as strength, speed, stamina and endurance, such as was exhiberted during the 'Senjutsu Training' arc, when Naruto fell from a balancing spike, during his senjutsu training with Fukasaku in order to master stillness; Naruto remained unharmed and felt hardly any pain. The strength and potency of the Sage's Taijutsu, Ninjutsu and Genjutsu are also increased drastically and dramatically in range, scope and power.

Naruto and Jiraiya are the only two human beings who are seen using it, with Naruto the only one of the two to master it. This is because Senjutsu training should never be undertaken lightly, as the risks involved are high: If the practitioning Sage should be unable to reign in control of their 'sage' chakra, the results can be disasterous; if the sage cannot draw on enough natural energy, then they cannot achieve 'sage mode'.

Likewise, if they should draw too deeply into nature, they will literally become apart of nature itself, their very body transmuting into solid stone, and transformed into the shape of a man-sized toad. There are many toad sculptures surrounding the fountain of the Shinobi-Toads Secret Toad Oil hidden on Myōbokuzan - all of these statues are the petrified remains of one-time apprentice Sages' who failed to master the art of senjutsu after taking in too much of natures power.

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.