Eugenics is the idea that you can engineer a better human population by breeding for certain genes. Since such a program would entail ranking human beings and the desirability of their genes, eugenics is widely considered unethical.
The term eugenics was coined in 1883 by Francis Galton, a cousin of Charles Darwin, and it comes from the Greek roots eu-, "good," and genos, "birth." Galton believed that the human race could be improved by encouraging people who have "good" genes to marry early and have lots of children, and discouraging people with "bad" genes from procreating at all. Nazi Germany provided a horrifying example of such a program at work, and eugenics is now seen as abhorrent.