noun
referee; umpire; head official
Sports term for the main official who enforces rules and makes final decisions during a match. In soccer and boxing, 'referee' is the typical translation; in baseball, 'umpire' is more common.
サッカーの試合で、主審がレッドカードを出した。
The referee showed a red card during the soccer match.
野球の主審はストライクか
The baseball umpire judges whether a pitch is a strike or a ball.
Compound of 主 (しゅ, 'chief') and 審 (しん, 'judge'). The reading is on'yomi for both characters.