The Japanese word senpai is a word directed at your senior in school or work.
Wikipedia defines Senpai as: "
Senpai (先輩?) and kōhai (後輩?) are an essential element of Japanese seniority-based status relationships, similar to the way that family and other relationships are decided based on age, with even twins being divided into elder and younger sibling. Senpai is roughly equivalent to the western concept of 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/protege in the West. 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."
In short, "senpai" is an honorific term used to signify that a person is an elder, senior or an upperclassmen. It is used to address a person who holds a higher position than a person of the same social class.
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You would call an upperclassmen this. Someone that is older than you, who you have respect for, and someone that has a higher social ranking than yourself.