assistant referee; assistant umpire
Used in sports contexts for an official who assists the main referee or umpire, such as in soccer, basketball, or martial arts.
サッカーの試合で副審がオフサイドを判定した。
The assistant referee judged the offside in the soccer match.
副審 is the assistant referee who supports the main official, in contrast to 主審, the head referee with final authority.
Compound of 副 (assistant, vice-) and 審 (judge, referee).