The referee showed a red card during the soccer match.
The baseball umpire judges whether a pitch is a strike or a ball.
A former pro player served as the referee for this match.
The players protested the referee's decision.
The referee blew the whistle and stopped the match.
主審 specifically means the chief or head referee/umpire in a game with multiple officials, whereas レフェリー is a general loanword for the referee.
副審 is the assistant referee who supports the main official, in contrast to 主審, the head referee with final authority.
線審 only judges boundary lines and offside calls, whereas 主審 is the chief official who controls the whole match.
Compound of 主 (しゅ, 'chief') and 審 (しん, 'judge'). The reading is on'yomi for both characters.