What is Social Darwinism?

Legal Definition
Social Darwinism is a name given to various phenomena emerging in the second half of the 19th century, trying to apply biological concepts of natural selection and survival of the fittest in human society. The term itself emerged in the 1880s. The term Social Darwinism gained widespread currency when used after 1944 by opponents of these earlier concepts. The majority of those who have been categorised as social Darwinists did not identify themselves by such a label.

Scholars debate the extent to which the various social Darwinist ideologies reflect Charles Darwin's own views on human social and economic issues. His writings have passages that can be interpreted as opposing aggressive individualism, while other passages appear to promote it. Some scholars argue that Darwin's view gradually changed and came to incorporate views from other theorists such as Herbert Spencer. Spencer published his Lamarckian evolutionary ideas about society before Darwin first published his theory in 1859, and both Spencer and Darwin promoted their own conceptions of moral values. Spencer supported laissez-faire capitalism on the basis of his Lamarckian belief that struggle for survival spurred self-improvement which could be inherited. An important proponent in Germany was Ernst Haeckel, which popularized Darwin's thought (and personal interpretation of it) and used it as well to contribute to a new creed, the Monist movement.
-- Wikipedia
Legal Definition
A theory that is used to explain success of social groups over other social groups. Only the strongest will survive, Cahrles Darwin and Origin of Species.