Nexus-6 is a type of andriod replicant found in "Do Androids Dream Of Electric Sheep?" and "Blade Runner" who possess superior strength, agility, and intelligence compared to the genetic engineers who created them. They were identical to humans.

These replicants were used in the off-world colonies as slave labor, mostly in hazardous exploration and colonization of other planets. They were declared illegal on Earth - under penalty of death.

To determine if a being is replicant or human, the law enforcement used the Voigt-Kampff empathy test. The problem was that Nexus-6 models were so advanced that the test had a difficult time delineating the emotional response. Rachel Rosen was the first Nexus-6 to be correctly identified by Rick Deckard as a replicant.

Those identified as replicants were done for.

This was not called execution.

It was called retirement.