Porous or hollow carbonaceous sphere-like particles (frequently in the size range of a few to several hundreds of µm diameter) formed during pyrolysis, also in the course of combustion, of carbonaceous liquid droplets (e.g. heavy fuel) or solid particles (e.g. coal).
Source:
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)