Wiktionary
alt. 1 (context military army English) 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 (context military navy English) A line of battle. 3 (context figuratively usually plural English) 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. n. 1 (context military army English) 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 (context military navy English) A line of battle. 3 (context figuratively usually plural English) 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.
WordNet
n. the line along which warring troops meet
Usage examples of "battle line".
He went fast, knowing that his careful battle line would be shredded by the oaks, but also knowing that any chance of finding an open Yankee flank was too compelling to be ignored.
He was also conscious that rank gave him the freedom to leave the battle line, except that the responsibility of command perversely decreed that he could not take that voluntary backward step.
The Colonel seemed remarkably calm as he aimed his revolver at the Northern battle line and methodically pulled the trigger.