front line

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun military A front, or a boundary between opposing positions.
• noun A site of a conflict, effort, or controversial matter of any kind.
• noun The site of interaction with outsiders, such as customers.
• noun euphemistic A low level.
• noun soccer attack, collectively the attackers or forwards.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the line along which opposing armies face each other