The application of evolutionary ideas — competition, survival of the fittest, natural selection — to human societies, used to justify inequality, empire, and eugenics.
Charles Darwin never endorsed it. Herbert Spencer coined 'survival of the fittest' after Darwin's Origin appeared, not before it. And yet Social Darwinism became the most influential pseudoscientific justification for inequality in modern history — applied to defend poverty, empire, racial hierarchy, and mass murder, all in the name of nature.