Eugenics is the scientifically erroneous and immoral theory of “racial improvement” and “planned breeding,” which gained popularity during the early 20th century. Eugenicists worldwide believed that they could perfect human beings and eliminate so-called social ills through genetics and heredity. They believed the use of methods such as involuntary sterilization, segregation and social exclusion would rid society of individuals deemed by them to be unfit.
The eugenics movement started long before the Holocaust and continued long after. The eugenics movement in Germany, and during the Holocaust took a particularly dark, outright proactive, turn. [Author’s note: I doubt if there were any “light” turns in this movement.]
In any discussion of eugenics, the terms unfit and undesirable constantly appear. Upon examination, these appear to be political rather than scientific definitions, which of course means that the definitions keep changing. It can be said that the Pharaohs practiced eugenics in Exodus by drowning Jewish infants. The Chinese practiced eugenics by killing (or more recently aborting) female children. More recently, in the 20th century, eugenics came to mean sterilization.
The Nazis of course carried this to extremes and performed outright killings.