Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements; chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed if dead plant matter decays into peat and over millions of years the heat and pressure of deep burial converts the peat into coal.
As a fossil fuel burned for heat coal supplies about a quarter of the world’s primary energy and is the largest source of energy for the generation of electricity. Some iron and steel making and other industrial processes burn coal.
The extraction and use of coal causes many premature deaths and much illness. Coal damages the environment; including by climate change as it is the largest anthropogenic source of carbon dioxide, 14 Gt in 2016 which is 40% of the total fossil fuel emissions. As part of the worldwide energy transition many countries have stopped using or use less coal.
The largest consumer and importer of coal is China. And China mines almost half the world's coal, followed by India with about a tenth. Australia accounts for about a third of world coal exports followed by Indonesia and Russia. 
The word originally took the form col in Old English, from Proto-Germanic *kula(n), which in turn is hypothesized to come from the Proto-Indo-European root *g(e)u-lo- "live coal". Germanic cognates include the Old Frisian kole, Middle Dutch cole, Dutch kool, Old High German chol, German Kohle and Old Norse kol, and the Irish word gual is also a cognate via the Indo-European root.