Regressive was a semi-derogatory term the Enarans applied to a minority of their population who rejected modern technology and cleaning methods such as radioseptics. The Regressives lived peacefully among the other Enarans, but they were greatly disdained in Enaran society.
A "voluntary resettlement" effectively eliminated the Regressives, because although the Enarans claimed the Regressives moved to a faraway colony, those who moved there were never heard from again. Korenna Mirell was witness to the fact that this injustice was being committed, but like most Enarans she denied the truth. Many years later, a dying Mirell transferred her memories to B'Elanna Torres, in the hope that one day the Enarans would have to own up to their actions. (VOY: "Remember")