诉讼 (sù sòng) is a formal verb meaning 'to file a lawsuit / to litigate', focusing on the entire legal process of bringing a case to court. It is more formal and broader than 起诉 (qǐ sù), which emphasizes the specific act of initiating a suit, and 告 (gào), which is a colloquial, everyday word for 'suing' someone. The noun 官司 (guān si) refers to the lawsuit itself as a matter or case.