This is the 1643rd most frequent Russian word.
донбасса
"Донбасса" is the genitive case of "Донбасс" (Donbas), referring to a region in eastern Ukraine rich in coal and heavy industry.
The word 'Донбасс' is used here as a proper noun referring to a geographical region.
Я приехал в Донбасс вчера вечером.
I arrived in Donbass yesterday evening.
'В Донбассе' utilizes the locative case to indicate that the discussion pertains to matters within the region of Donbass.
Мы обсудили текущую ситуацию в Донбассе.
We discussed the current situation in Donbass.
The genitive form 'Донбасса' is used to describe something belonging to the region, in this case, its history.