WordNet (r) 3.0 (2006):

sniper
    n 1: a marksman who shoots at people from a concealed place

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "sniper":
   Nimrod, Zouave, archer, artilleryman, bersagliere, bowman,
   cannoneer, carabineer, chasseur, crack shot, dead shot, deadeye,
   dogface, doughfoot, expert rifleman, foot soldier, footslogger,
   fusileer, good shot, grenadier, grunt, gun, gunman, gunner, hunter,
   infantryman, light infantryman, marksman, markswoman, musketeer,
   paddlefoot, rifle, rifleman, sharpshooter, shooter, shot,
   targetshooter, toxophilite, trapshooter