The line along which warring troops meet WordNet 3.0 & Open English WordNet

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. Wiktionary

The notional line on which forces are arrayed on either side. Wiktionary (translation)