assistant referee

US /əˌsɪs.tənt ˈref.ə.ri/
UK /əˌsɪs.tənt ˈref.ə.ri/
"assistant referee" picture
1.

an official who assists the referee in a soccer match, typically by indicating when the ball is out of play or when a player is offside

:
The assistant referee raised the flag for offside.
The decision was confirmed by the assistant referee.