Porous or hollow carbonaceous sphere-like particles (frequently in the size range of a few to several hundreds of µ m diameter) formed during pyrolysis, also in the course of combustion, of carbonaceous liquid droplets (e.g. heavy fuel) or solid particles (e.g. coal).
PAC, 1995, 67, 473. 'Recommended terminology for the description of carbon as a solid (IUPAC Recommendations 1995)' on page 480 (https://doi.org/10.1351/pac199567030473)