Senpai (先輩) and kōhai (後輩) are an essential element of Japanese age-based status relationships, similar to the way that family and other relationships are decided based on age, with even twins being divided into older and younger sibling. Senpai is roughly equivalent to the western concept of "mentor", while kōhai are roughly equivalent to "acolytes". A young person may be considered the senpai of an older person at times if the older person entered an organization, such as a company, at a later time than the younger person did. Senpai is often seen romanized as "sempai" because it is pronounced that way (the Japanese "n" (ん) is pronounced as "m" when it comes before certain consonant sounds, like "p").
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.
These terms appear frequently in anime and manga. Romantic crushes on one's senior are a popular theme in shōjo (young female stories) and romantic stories. In translated anime and manga, senpai/kōhai are more commonly retained in fansubs and scanlations, as official translations tend to use familiar English terms.
In Japanese martial arts, the term senpai generally refers to the highest ranked student in a club who is not yet a black belt. They are expected to assist the sensei with younger or less experienced students.