linesman

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Football An official who marks the downs and the position of the ball and watches for certain violations from the sidelines.
• noun An official in various court games whose chief duty is to call shots that fall out of bounds.
• noun An official in soccer and other field games who indicates when the ball has gone out of bounds.
• noun Either of two officials in ice hockey who call icing and offsides, conduct face-offs, and call penalities.

from The Century Dictionary.

• noun Milit., a private in the line; an infantryman.
• noun In Amer. foot-ball, one of the forwards; also, the man who has charge of the measurements of the distance gained and assists the umpire in various ways.
• noun A workman engaged in the erection or maintenance of the line-wires of a telegraphic or telephonic system, or of electric circuits of any sort.

from the GNU version of the Collaborative International Dictionary of English.

• noun the official (in tennis or soccer) who watches the lines.
• noun a person who installs or repairs electrical or telephone lines.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun soccer, dated An assistant referee.
• noun tennis An official who calls shots that pitch out of bounds, and judges foot faults.
• noun ice hockey An official whose primary task is to watch the blue line and determine when there has been an offside.
• noun American football An official whose primary task is to determine whether there has been a line of scrimmage violation.
• noun A person employed to work on electrical lines; a lineman.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun official (in tennis, soccer, football, etc.) who assists the referee in some way (especially by watching for out of bounds or offside)
• noun a person who installs or repairs electrical or telephone lines