Fly ash, flue ash, or coal ash is a solid fraction of carbonaceous material (carbon-rich) produced by incomplete combustion of fuels in air or oxygen. The particle size ranges from submicrometer particles to large multi-thousand-micrometer particles (also known as silica). Ash that falls to the bottom of the boiler's combustion chamber (commonly called a firebox) is called bottom ash. Fly ash can be captured by electrostatic precipitators or other particle filtration equipment before it reaches the chimneys.

Coal fly ash is the by-product of coal combustion and contains a great number of impurities, including silicon dioxide (SiO2), calcium oxide (CaO), calcium hydroxide (Ca(OH)2), aluminium oxide (Al2O3), and ferric oxide (Fe2O3). Fly ash has been used in concrete as an aggregate due to its high alumina content.