What is a sannin?

Answer 1

A sannin is one of the three legendary ninja such as Jaraiya, the toad hermit/sage/pervy sage, Lord Orochimaru, and Lady Tsunade, the legendary sucker.

They got their names because they were tough enough to get to and hold their own against Hanzou, the leader of the village hidden in the rain. Though they didn't beat him, they lasted a good while, and given their age and experience, they were legendary, as no one before them were able to do even that. Since then, however, one of Jaraiya's orphan pupils in that village grew up and killed Hanzou, which not even jaraiya could do, and Jijraiya thought that it was not even possilbe!

Sannins are S class ninjas, the toughest known in this fictional world of Naruto, said to be at Kage level, and the 3rd Hokage (each of the Five Great Shinobi Country's Ninja village has a specific prefix for their kage and Konoha's is "Ho") of Konohagakure, the village in the leaves, was the one who taught the Sannin.

However, these Sannin are all about 50 years old, and they have apprentices now, for they are not going to be on the very top of their game for much longer, as Tsunade has Shizune (and later Sakura and Ino), Jiraiya has Pain, the one who killed Hanzou, and Naruto, the son his pupil, the 4th Hokage (leaf village) with Orochimaru having multitudes of underlings, like the Sound genin (all dead, presumably), the Sound 4, and all of his base henchman, test subjects, and prisoners.

The new Sannin will most likely be Naruto for Jiraiya,Sasuke(?) for Orochimaru, and Sakura for Tsunade (who gets a HUGE jump in abilites after her training).