Senpai (先輩) is a Japanese term (sometimes an honorific) for a person in a club or other organization, including a school or college, who is a senior, in other words a member of a year above, and mutually recognized as such. The junior equivalent is called kōhai (後輩). Senpai is often seen romanized as Sempai, in accordance with the original version of the Hepburn romanization system.
Senpai and kōhai 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.
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 are used widely among otaku and in martial arts as well as appearing frequently in anime and manga. Romantic crushes on senpai are fairly common in Japan, and this is reflected as a popular fetish in shōjo and romantic stories. In translated anime and manga, senpai/kōhai are more commonly used in fansubs and scanlations, as official translations tend to use familiar English terms.