The South (or the Global South) is a collective term for all the
poor countries of the world, since most developed countries exist in the temperate zone of the
Northern Hemisphere. Poverty in this case is usually defined by the level of physical and human infrastructure development.
Alternative terms have included Least Developed Countries, Low-Income Countries, Developing Countries, the Majority World and the Third World, which dropped out of common vernacular following the victory of the First World over the Second in the early 1990s.
The term is somewhat loaded. It suggests that geographical location plays a deciding factor in a country's level of economic development, and this would help endorse the notion that the imperialist or neo-imperialist rich became rich by colonising and ripping off the poor (dependency theory). Spatial metaphors are often subjectively used to denote an unequal relationship between whoever is 'on top' and those who are at 'the bottom', so it is perhaps not suprising that the term originated in internationalist circles.
Aside from being inaccurate if you consider the location of wealthy Australia in the South or impoverished Mongolia in the North, it ignores important differences within each groups, such that several South countries have advanced to the stage that the majority of their citizens enjoy the living standards of the North.