What do you call a hot piece of material that remains after a substance has partially burned?

The term for a hot piece of material that remains after a substance has partially burned is "ember." An ember is a glowing, hot piece of wood or coal that continues to emit heat and may still be capable of igniting nearby materials. This is characteristic of embers, as they are typically formed during the smoldering phase of combustion when a material has burned down but is still capable of producing heat and light.

Coal, while it can also remain hot after burning, usually refers to a solid fuel that has not necessarily gone through the combustion process to the same extent as an ember. Similarly, cinders are generally the ash-like remnants that result after material has burned, which are cooler and often do not glow. Charcoal is created through the pyrolysis of wood and does not spontaneously form from burning; it represents a different stage of the burning process. Understanding these distinctions helps clarify why "ember" is the accurate choice in this context.