Senior — anyone who started before you.
School, university, company, band: whoever arrived first is your 선배, and you are their 후배. It is used as a title on its own or after a name, and it quietly sets who speaks casually to whom. Being one year ahead is enough.
Written 先輩 in Chinese characters — the same two characters Japanese reads as senpai.
This is a real hierarchy marker, not a cute nickname. Skipping it with someone genuinely senior reads as rude.
Reading a definition isn't the same as saying it to someone. Lingo Kaiava is a voice tutor set to Korean — it uses 선배 the way locals do, corrects you in the moment, and never "fixes" the way real people actually talk into textbook Korean.
Free to start. No signup for your first conversation.