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Old English sot "soot," from Proto-Germanic *sotam "soot" (cf. Old Norse sot, Old Dutch soet, North Frisian sutt), literally "what settles," from PIE *sod-o- (cf. Old Church Slavonic sažda, Lithuanian suodžiai, Old Irish suide, Breton huzel "soot"), suffixed form of root *sed- (1) "to sit" (see sedentary).
A black, powdery substance that consists mainly of carbon and is formed through the incomplete combustion of wood, coal, diesel oil, or other materials. Because it absorbs energy from sunlight rather than reflecting it, soot is believed to be a cause of global warming, especially when it settles on snow and ice, reducing their reflectivity. Soot particles in the air are a contributing factor in respiratory diseases.