The South is a term usually used by inhabitants of the North to refer to the rest of Westeros. They hold that the South includes all the lands from the Neck to the Summer Sea, including the Vale, Riverlands, Westerlands, Crownlands, Stormlands, the Reach and Dorne. Natives of those regions are known as 'Southrons' by the northmen.
This usage reflects the geographic make-up of Westeros, with the North making up the northern half of the continent and the South the southern half. However, natives of the southern regions usually count things by population, where the North, despite its vast geographic size, is less populous than a much smaller region such as the Reach and is thus not held to be any more important. As a result, the term is not in wide use outside of the North.