Sensei:

Sensei is a Japanese term that indicates "the one who was there before me, who guarantees the knowledge and the experience of a technique or know-how", or in a more specific way a master who gives his education to Sempaï or Kohaï (students).

Sempaï and Kohaï:

In the Japanese culture, Sempaï is the advanced student and Kohaï is a new (or younger) student. The Sempaï has guardian's role of the Kohaï, and he has a relay role of the education of the sensei (the teacher); in return, the Kohaï must hold a high respect for the Sempai. Two students of the same seniority, are known as Dohaï to one another.

In Budô (martial arts), the older students are there to guide the new students. The notion of Sempaï or Kohaï depends exclusively on the seniority of the student in the art, not on their age or rank.