WordNet (r) 3.0 (2006):

front line
    n 1: the line along which opposing armies face each other [syn:
         battlefront, front, front line]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "front line":
   DMZ, aceldama, advance guard, airhead, avant-garde, battle line,
   battle site, battlefield, battleground, beachhead, bridgehead,
   combat area, combat zone, enemy line, farthest outpost, field,
   field of battle, field of blood, firing line, first line,
   forefront, front, front rank, front-runner, killing ground,
   landing beach, line, line of battle, outguard, outpost, pioneer,
   point, precursor, railhead, scout, seat of war, shambles,
   spearhead, the front, theater, theater of operations,
   theater of war, van, vanguard, zone of communications