linesman

Concise Oxford English Dictionary © 2008 Oxford University Press:
linesman/ˈlʌɪnzmən/

▶noun (pl. linesmen)

• 1 (in games played on a field or court) an official who assists the referee or umpire, especially in deciding whether the ball is out of play.

• 2 Brit. a person who repairs and maintains telephone or electricity power lines.