As sedimentary rock formed deep underground, coal has high heterogeneity and complexity.
Some experts provided a definition of coal, namely:
1. Spackman (1958): Coal is a carbon-solid object with a certain maceral composition.
2. The International Hand Book of Coal Petrography (1963): Coal is a flammable sedimentary rock, formed from remnants of plants in varying levels of preservation, bound by the compacting process and buried in earth cavity at varying depths, from shallow to deep.
3. Thiessen (1974): Coal is a complex solid object, consisting of various chemical elements or is a very complex organic solid.
4. Achmad Prijono, et al. (1992): Coal is a solid hydro-carbon fuel which is formed from plants in an oxygen-free environment and is subjected to the effect of temperature and pressure over a period of very long time.
The definition of coal can be summarized from the above source as: "Coal is the sediment of solid hydrocarbon organic fuel, which is formed from plants that have undergone biochemical, chemical and physical decay under oxygen-free conditions that take place at certain pressures and temperatures over a period of very long time "