front line

US: (frunt′līn′)

WordReference Random House Unabridged Dictionary of American English © 2025

front′ line′, 

1. front (def. 9).
2. the visible forefront in any action, activity, or field:
   TV reporters are constantly involved in the front line of events.
3. Sport [Football.] See front four. 
4. Sport [Basketball.] See front court (def. 2b).
• 1915–20

front-line  (frunt′līn′), adj. 

1. Military located or designed to be used at a military front line:
   a front-line ambulance helicopter.
2. of, pertaining to, or involving the forefront in any action, activity, or field:
   a front-line TV reporter.
3. highly experienced or proficient in the performance of one's duties.

• 1910–15

Collins Concise English Dictionary © HarperCollins Publishers::

front line noun

1. the most advanced military units or elements in a battle
2. frontline, front-line (modifier) of, relating to, or suitable for the front line of a military formation: frontline troops
3. of or relating to a country bordering on or close to a hostile country or scene of armed conflict: leaders of the frontline states attended the summit