煤灰 (méi huī) literally combines 煤 (coal) and 灰 (ash), meaning the fine gray or black powdery residue left after coal is burned. It is a byproduct of coal combustion in power plants, industrial furnaces, or household stoves. Unlike wood ash, coal ash contains trace heavy metals and minerals, making it chemically distinct and often subject to environmental regulation.

In everyday usage, 煤灰 appears in contexts related to energy production, pollution control, construction (where it’s sometimes used as a supplementary cementitious material), and environmental science. Though visually similar to other ashes, it carries specific technical and ecological connotations — for instance, fly ash and bottom ash are two common subtypes classified by how they’re collected during burning. The term is neutral in register but leans formal or technical; it rarely appears in casual conversation unless discussing heating, waste, or local air quality.