Senpai and kōhai
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Senpai (先輩) and kōhai (後輩) are terms applied to the mentor system in wide use in Japanese culture; often found at all levels of education, in sports clubs, businesses, and informal or social organizations. The relationship is an essential element of Japanese seniority-based status relationships, similar to the way that family and other relationships are decided based on age, in which even twins may be divided into elder and younger siblings. The senpai is roughly equivalent to the Western concept of a mentor, while kōhai is roughly equivalent to protégé, though they do not imply as strong a relationship as these words mean in the West. More simply, these may be translated as senior and junior, or as an elder compared with someone younger in the family/company/organization; the terms are used more widely than a true mentor/protégé in the West and are applied to all members of one group that are senior (the senpai) to all the members of another group (the kōhai). There is usually no average separation in age between a senpai and his or her kōhai.
A junior student will often refer to senior students as "senpai", and alumni will often refer to alumni from earlier classes as "senpai". This holds true particularly if events bring them together later on, such as joining the same company, serving on a board together, or simply being in a club or parent's organization at the same time.
On rare occasions, a younger person may also be considered the senpai of an elder person if circumstances dictate—such as if the elder entered an organization or company at a later time than the younger did.
Social roles 
In a Japanese school sports club, such as a baseball team, the kōhai are usually expected to perform various menial tasks for the senpai including washing clothes and cleaning. The kōhai may not be allowed to play the sport at all or have only limited opportunities to do so until they become senpai.
More than simple seniority, senpai implies a relationship with reciprocal obligations, somewhat similar to a mentoring relationship. A kōhai is expected to respect and obey their senpai, and the senpai in turn must guide, protect, and teach their kōhai as best they can. Senpai/kōhai relationships generally last for as long as the two people concerned stay in contact, even if the original context in which the senpai was senior is no longer relevant.
See also 
- "Sempai-Kohai" on sci.tech-archive.net
- "Re: Correcting sempai" on webdocs.cs.ualberta.ca
- "Japanese honorifics and situation semantics" on ACM Digital Library