煮沸 means 'to boil' — specifically, to heat a liquid until it reaches its boiling point and begins bubbling vigorously. The first character 煮 (zhǔ) means 'to cook by boiling or simmering', while 沸 (fèi) means 'to boil over' or 'to bubble up', emphasizing the active, turbulent state of boiling. Together, they form a compound verb that stresses the completion of the boiling process, not just heating. It’s commonly used in cooking instructions, water purification contexts, and health-related advice (e.g., boiling water before drinking).

This word is more formal and precise than the simpler verb 煮 (zhǔ), which can mean 'to cook' generally. 煮沸 often appears in written instructions, safety guidelines, or scientific explanations rather than casual speech. It implies intentional, full boiling — for example, to kill bacteria or extract flavors — and is rarely used metaphorically.