The article is devoted to the analyses of theoretical approaches towards the definition of the concept «region» in Russian and foreign researches. Research studies of a region as a socio-economic system appears to be sufficiently conventional within the modern socio-economic and geographic science, however the united approach towards theoretical evaluation of the category, its definition and its forming subsystems does not still exists. Region is considered to be a complex spatially integral united system, which is represented by a local territorial complex of interconnected social and economic subsystems, determined by the definite production structure, peculiarities of the social infrastructure and human resources, taking into consideration the place and role of the particular region in the social and economic complex of the country. The authors put emphasis on a topological approach towards the definition of the concept «the regional social, economic and geographical system». The model of the regional social, economic and geographical system is important for each entity of the federal state and it is developed, taking into consideration peculiarities and characteristics of the subjects. Thus, the region has constitutionally fixed competence and authority. There are three levels of functional and structural subsystems, which are macro-, mezo- and microlevels. The levels of the social, economic and geographical system (SEGS) coincide with the state structure existing in Russia, due to its role in the forming base of all territorial social and economic systems