1.
substitute; reserve player; alternate
A person who fills in for someone who is absent or unable to participate. Commonly used in sports for reserve or bench players, and in elections or exams for alternates.
Usage
補欠 is used in several distinct contexts:
1. SPORTS: A reserve or bench player who may be called on if a regular player cannot play.
2. EXAMS/ADMISSIONS: 補欠合格 means being placed on a waitlist; the candidate is accepted if someone who passed declines their spot.
3. ELECTIONS: 補欠選挙 (by-election) is held when a seat becomes vacant.