Someone senior to you at school or work.
Defined purely by when you joined, not by age or rank. Its counterpart is 後輩 (kouhai), the junior. The relationship carries real obligation in both directions: the senpai looks out for you, and you defer. Often used in place of a name.
The English internet turned it into a joke about crushes. In Japan it is an ordinary, hierarchical word you use daily.
Reading a definition isn't the same as saying it to someone. Lingo Kaiava is a voice tutor set to Japanese — it uses 先輩 the way locals do, corrects you in the moment, and never "fixes" the way real people actually talk into textbook Japanese.
Free to start. No signup for your first conversation.