In sports played on a marked field or court, a linesman is an official who stands by the side line and watches whether the ball — or a player — has gone out. You meet the word in football (soccer), hockey, and tennis. Though actually, names have been shifting. In football, the job is now officially “assistant referee,” and younger commentators often say that; older fans still say linesman, and everyone knows what it means. In tennis, people also say line judge. The word still sounds like “a man,” but women do the job too; some organisations say linesperson. It is a sports-official word, not a classroom word. Do not confuse it with lineman, which is a player on the American-football line, or someone who works on power lines.