Wiktionary
line judge
n. (context tennis English) An official who watches the lines of the court who judges whether the ball has landed in or out and also declares foot faults.
WordNet
line judge
n. football official who assists the referee by keeping track of the official time during the game
Usage examples of "line judge".
Who would have thought it: Matern, who had come with black dog to judge, makes out fine as a referee and trains his mutt to be a line judge: Pluto barks at every foul.