吐痰 is a verb meaning 'to spit phlegm' — the act of expelling mucus from the respiratory tract through the mouth. The first character 吐 (tǔ) means 'to spit', 'to vomit', or 'to eject', while 痰 (tán) specifically refers to 'phlegm' or 'sputum', the thick mucus produced in the lungs and airways during illness or irritation. Together, they form a compound verb that emphasizes the physiological action of clearing respiratory secretions.

This term is commonly used in health-related contexts — such as medical advice, public hygiene campaigns, or discussions about colds and respiratory infections. It carries a neutral-to-slightly-negative connotation because spitting phlegm in public is widely discouraged in China and many other cultures for reasons of sanitation and social etiquette. While it describes a natural bodily function, the word often appears in warnings or instructions about proper disposal (e.g., using tissues) rather than casual description.