'余热' (yú rè) literally means 'remaining heat' — combining 余 ('remaining, leftover') and 热 ('heat'). It functions as a noun with two primary meanings: first, in physics and engineering, it refers to residual heat — thermal energy left after a process (e.g., exhaust heat from machinery); second, figuratively, it denotes lingering influence or continued impact, especially of a person’s reputation, effort, or presence after retirement, departure, or even death.

The term is neutral in register but leans formal or technical. In medical contexts, it can describe low-grade fever persisting after an illness begins to resolve — a subtle but clinically meaningful sign. In social or cultural usage, it often appears in phrases like '发挥余热' ('to put one’s remaining energy/ability to use'), commonly said of retirees contributing expertise. Its dual physical and metaphorical nature makes it rich in nuance, relying heavily on context to distinguish literal versus figurative senses.