The core of "Social Darwinism" is applying evolutionary biology — most notably, the idea of "survival of the fittest" (a phrase coined by Spencer, not by Darwin) — to sociology, economics and politics, on the assumption that social progress will result from conflict between groups and individuals as superior out-competes inferior.
The term "Social Darwinism" was coined in Europe in 1877 in order to stigmatize various social theories then emerging in England and the United States. Popularized in North America after 1944 by historian Richard Hofstadter, it's been applied, fairly or not, to such writers as Herbert Spencer, Darwin's cousin Francis Galton (the founder of the "eugenics" movement to breed better humans) and Thomas Malthus. Apparently never used by actual "Social Darwinists," it was certainly not used by Charles Darwin himself, though he may have been sympathetic to the ideas it labels.
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