1. the foremost line of troops in active combat; the area of direct military engagement
Usage: usually plural or used with singular verb; military context
2. the most active or prominent position in any struggle, campaign, or movement
Usage: figurative use; usually plural or used with singular verb

Examples

• Soldiers on the frontlines faced intense enemy fire.
• The general visited troops stationed at the frontlines.
• Climate activists are on the frontlines of environmental change.
• Doctors are on the frontlines of the pandemic response.
• The frontlines of the civil rights movement were marked by courage and sacrifice.
• Journalists reported from the frontlines of the conflict.
• Teachers are on the frontlines of education reform.