What is the front lines?

The front lines is the up close and first hand experience of any human on the battlefront. Also known as the line where two or more opposing armies face each other whether it be air, land, or sea. The front lines is also the place where all the bullets, bombs, etc, are flying. According to merriam-webster the front lines is a military line formed by the most advanced tactical combat units. The Free Dictionary says that it's a region where a battle is being (or has been) fought. Then the book, Up Front, by Bill Mauldin descriobes the front lines as boring, unless there is some action going on.