The Karoo ( ; from a Khoikhoi word, possibly garo "desert") is a semi-desert natural region of South Africa. There is no exact definition of what constitutes the Karoo, and therefore its extent is also not precisely defined. The Karoo is partly defined by its topography, geology, and climate — above all, its low rainfall, arid air, cloudless skies, and extremes of heat and cold. The Karoo also hosted a well-preserved ecosystem hundreds of million years ago which is now represented by many fossils.