An oblast is a region, an administrative division in Ukraine, Belarus, and Russia. It is roughly equivalent to 'state' in American terms, or 'county' in the UK terms, or 'departement' in French terms.
It is pronounced with a soft 't' at the end, like in 'tea'. The stress is usually on the 'o'.
What is noticeable since the beginning of the war in Ukraine (NB: not 'the' Ukraine), is that Western media have adopted the local term 'oblast' and not 'region'.
Proof, from the BBC:
"Russia suffered a serious defeat around Kyiv and had to pull back. But it has not lost the war, and is grinding out territorial gains in the east and south. Since they left the Kyiv Oblast the damage they have done is clearer than ever."