复发率 (fù fā lǜ) is a medical term meaning 'relapse rate' — the percentage or proportion of patients whose disease returns after initial treatment and apparent recovery. The word combines three characters: 复 (fù, 'again' or 're-'), 发 (fā, 'to occur' or 'to break out'), and 率 (lǜ, 'rate' or 'ratio'). Together, they literally mean 'the rate at which something occurs again'. It is used almost exclusively in clinical, epidemiological, and public health contexts to quantify how often a condition reappears following remission or discharge.

This term appears frequently in research papers, doctor–patient discussions about prognosis, and health policy reports. It is typically expressed as a percentage (e.g., '15%') or a decimal fraction (e.g., '0.23'). Unlike general terms like 'recurrence', 复发率 specifically denotes a measured, statistical frequency — not just the event itself. It applies mainly to chronic or episodic illnesses such as cancer, depression, tuberculosis, or autoimmune disorders.