Which term refers to partially incinerated matter in combustion products?

The term "fly ash" specifically refers to the fine particulate matter that is produced during the combustion of materials, such as coal or wood, which becomes airborne in the heat of the process. Fly ash is a byproduct that can be carried away in the exhaust gases, settling out of the air afterward. This is why it is characterized mainly by its fine, powdery texture and composition that usually consists of silica, alumina, and oxides of iron.

In the context of cremation and other combustion processes, fly ash can also arise from the incineration of human remains, which results in the combustion of organic material and the production of particles that are small enough to be suspended in air. These particles are often collected using filtration systems to prevent them from entering the atmosphere.

This understanding of fly ash supports its designation as the correct answer in this scenario, distinguishing it from other terms that refer to different forms of ash or waste matter. For example, waste ash typically represents residual ash left behind that does not become airborne, while cinders refer to larger fragments that are not effectively carried away, and debris generally denotes a broader category of leftover material that does not specifically define the particulate nature tied to combustion byproducts.