noun
charcoal briquette
A traditional Japanese fuel made from charcoal powder pressed into a round or oval shape, often used in hibachi or kotatsu. Now somewhat old-fashioned.
昔は冬になると炭団を火鉢で使ったものだ。
In the old days, people used charcoal briquettes in a hibachi during winter.
木炭 is general charcoal, often lump charcoal from wood, while 炭団 is specifically a pressed briquette made from charcoal powder.
Compound of 炭 (sumi, 'charcoal') and 団 (dan, 'round mass, lump'). The reading たどん is a sound shift from たんだん.