案件 (àn jiàn) is a formal noun meaning 'legal case' or 'criminal case'—a matter under investigation or adjudication by police, prosecutors, or courts. The character 案 (àn) originally meant 'desk' or 'case file', and by extension came to signify an official matter or incident; 件 (jiàn) is a general measure word for matters, events, or items, reinforcing the sense of a discrete, identifiable incident. Together, they denote a legally defined matter with procedural significance.

This term appears almost exclusively in official, judicial, or news contexts—not in casual speech. You’ll encounter it in headlines (e.g., '重大案件破获'), police reports, court documents, and legal discussions. It implies seriousness, formality, and institutional involvement—never used for minor disputes or personal disagreements. While 案 can stand alone in compounds like 案发 (‘case occurred’) or 案情 (‘case details’), 案件 is the standard, standalone noun for ‘case’ in legal usage.