front line

Short definition from Wiktionary

Noun

1. 1
   A front, or a boundary between opposing positions.
2. 2
   A site of a conflict, effort, or controversial matter of any kind.
3. 3
   The site of interaction with outsiders, such as customers.
4. 4
   A low level.
5. 5
   attack, collectively the attackers or forwards.

Adjective

1. 1
   Alternative spelling of front line.