Oblast

Oblast is a type of administrative division in most Slavic countries and some countries of the former Soviet Union. The word "oblast" is used in English as well, but it is often translated as "area", "zone", "province", or "region". The last translation may lead to confusion, because the subdivision of "oblast" is called "raion" which can be translated as "region" or "district" as well.
Oblasts are a type of administrative division of Belarus, Bulgaria, Kazakhstan, Kyrgyzstan, Russia, Ukraine, it was also used by the Soviet Union which does not exist anymore. Official terms in successor states of the Soviet Union differ, the majority (except Turkmenistan, Tajikistan and the Baltics) still use a cognate of the Russian term, e.g. voblast (voblasts, voblasts' , ) is used for provinces of Belarus, and oblys (plural: oblystar) for provinces of Kazakhstan.
Oblasts of Bulgaria.
Since 1997, Bulgaria has been divided into 28 "oblasti", usually translated as "provinces". Before, the country was divided into nine bigger units, also called "oblast".
Oblasts of Ukraine.
Ukraine is divided into 24 oblasts.
Oblasts of Byelorussia.
Byelorussia is divided into 6 oblasts.
Oblasts of the Russian Empire.
In the Russian Empire, oblasts were considered to be administrative units and were included as parts of Governorates General or krais. The majority of then-existing oblasts were on the periphery of the country or covered the areas where Cossacks lived.
Oblasts of the Soviet Union.
In the former Soviet Union, oblasts were one of the types of administrative divisions of the union republics. As any administrative units of this level, oblasts were composed of districts ("raions") and cities/towns directly under oblasts' jurisdiction. 
The names of oblasts did not usually correspond to the names of the historical regions, as they were created as purely administrative units. With a few exceptions, Soviet oblasts were named after their administrative centers.
Terms used to describe oblasts in post-Soviet countries.
The oblasts in other post-Soviet countries are officially called:
"Viloyat" and "welayat" come from the Arabic language term "wilāya" (ولاية)
Oblasts of Russia.
According to the Constitution of Russia, oblasts are considered to be subjects of the Federation, which is a higher status than that of administrative units they had within the Soviet Union. The federal subject status gives the oblasts some degree of autonomy and gives them representation in the Federation Council.
Oblasts of Yugoslavia.
Oblasts were administrative units of the Kingdom of Serbs, Croats and Slovenes between 1922 and 1929. During that period, the country was divided into 33 oblasts. In 1929, oblasts were replaced with larger administrative units known as "banovinas".
During the Wars] that split Yugoslavia several Serbian Autonomous Oblasts were formed in Croatia and Bosnia and Herzegovina. These oblasts were later merged into the Republic of Serbian Krajina and the Republika Srpska.