Noun

1. A black rock formed from prehistoric plant remains, composed largely of carbon and burned as a fuel.
2. A piece of coal used for burning. Note that in British English the first of the following examples would usually be used, whereas in American English the latter would.

   Put some coals on the fire.

   Put some coal on the fire.

3. A type of coal, such as bituminous, anthracite, or lignite, and grades and varieties thereof.
4. A smouldering piece of material.

   Just as the camp-fire died down to just coals, with no flames to burn the marshmallows, someone dumped a whole load of wood on, so I gave up and went to bed.

Verb

1. To take in coal; as, the steamer coaled at Southampton.
2. To be converted to charcoal.

   • 1957: As a result, particles of wood and twigs insufficiently coaled are frequently found at the bottom of such pits. — H.R. Schubert, History of the British Iron and Steel Industry, p. 18.