LINESMAN

[noun] [C] [plural] -men - an official at a sporting event who is responsible for deciding when the ball has crossed the line that marks the edge of the playing areaThe linesman claimed the ball was out, but the umpire decided it was in.A linesman also helps the official in charge of a game to decide when a player has broken a rule.The referee consulted the linesman before sending the captain off the field.