front line Definition

1a. In a war, particularly when fought on the ground: the area of a front (noun 7) where soldiers are physically closest to the enemy;
1b. In a war, particularly when fought on the ground: belonging or relating to the front line.

Example: front-line soldiers
2. That area in any concern where the important pioneering work is going on.
3. Relating to a state bordering on another state in which there is an armed conflict, and often inevitably involved in that conflict.

Form: front-line