battle line

Short definition from Wiktionary

Noun

1. 1
   The notional line on either side of which opposing forces are arrayed, and at which they meet in battle; a fortified line, short of the line where battle is expected, and behind which forces are arrayed; a forward line of infantry or other force, prepared for battle.
2. 2
   A line of battle.
3. 3
   Any of those parts of a contest which, as a result of strategic decisions on both sides, are most closely fought and will likely determine the contest's course and outcome.
4. 4
   Alternative form of battleline.