Seoul is traversed by the Hangang river which flows from east to west. The whole area north of the river is called Gangbuk, and the south of the river is called Gangnam. (Korean word ‘gang’ means river, ‘buk’ is north, and ‘nam’ is south.) So, this is the wider definition of Gangnam. Then, there is an administrative district named Gangbuk-gu in the Gangbuk area, and an administrative district named Gangnam-gu in the Gangnam area. This is the narrow definition. And there is a third definition. When Gangnam is mentioned, it usually refers to the three adjacent districts of Seocho-gu, Gangnam-gu, and Songpa-gu. These three districts have many characteristics in common, the first of which is the skyrocketing costs for housing.
Image by Wikipedia(https://en.wikipedia.org/wiki/File:Seoul_Gangnam-gu.svg), CC BY-SA 2.0 license.