起诉 (qǐsù) is the formal legal term for 'to sue' or 'to bring a lawsuit against' — it specifically refers to initiating a civil or criminal case in court. It is more technical than 告 (gào), a broader everyday verb for 'to sue' or 'to tell/report', and unlike 控告 (kòng gào), which emphasizes accusing someone of a crime, 起诉 focuses on starting the lawsuit. 诉讼 (sùsòng) more often refers to the litigation process, not just the initiation.