A raion (also rayon) is a type of administrative unit of several post-Soviet states (such as part of an oblast). The term is from the French "rayon" (meaning "honeycomb, department"), which is both a type of a subnational entity and a division of a city, and is commonly translated in English as "district".
The term "raion" also can be used simply as a kind of administrative division without anything to do with ethnicity or nationality. A raion is a standardized administrative entity across most of the former Soviet Union and is usually a subdivision two steps below the national level. However, in smaller USSR republics, it could be the primary level of administrative division ([[Administrative divisions of Armenia], Administrative divisions of Azerbaijan). After the fall of the Soviet Union, some of the republics dropped raion from their use (Armenia).
In Bulgaria, it refers to an internal administrative subdivision of a city not related to the administrative division of the country as a whole, or, in the case of Sofia municipality a subdivision of that municipality.
More information on the Wikipedia page