The term COAL UNIT can be broken down into two crossword solutions: LUMP (4 letters) and TON (3 letters). LUMP is a term used to describe a large, irregularly shaped piece of coal. A TON is a unit of measurement used to measure the weight of a substance, in this case coal. Coal is a combustible black or brownish-black sedimentary rock that is composed mostly of carbon and hydrocarbons. It is widely used as a fuel source for electricity generation and industrial processes.