立案 (lì àn) literally means 'to establish a case' — 立 (lì) means 'to set up, to establish', and 案 (àn) means 'case' or 'file', especially in legal or administrative contexts. Together, they form a formal verb meaning 'to officially file a case' — the procedural step when authorities (e.g., police, courts, or regulatory agencies) formally accept and register a reported incident for investigation or adjudication.

This term is used exclusively in official, institutional settings: it marks the beginning of a formal process, not just reporting or complaining. It implies that preliminary evidence or grounds have been reviewed and deemed sufficient to proceed. You’ll encounter 立案 in news reports about criminal investigations, court announcements, or government service guidelines — never in casual conversation. It carries legal weight and often triggers statutory timelines, such as deadlines for investigation or prosecution.