A movement among the Enarans that refused modern technology, and whose members were rounded up and killed a generation before the Enarans met Voyager, an atrocity later denied.
The Regressives were Enarans who chose to live simply and turned their backs on the advanced technology the rest of their society embraced. The majority came to see them as dirty, dangerous and backward, and the authorities began moving them to a supposed colony elsewhere.
Their fate is revealed in Remember. While Voyager carries a party of Enarans home, B'Elanna Torres has vivid dreams in which she is Korenna Mirell, a young woman in love with a Regressive named Dathan Alaris. Korenna's father persuades her that the Regressives are a threat, and she betrays Dathan, who is publicly executed. The resettlement was in fact extermination.
The dreams are memories passed on by the dying Jora Mirell, Korenna in old age. Her companions deny everything, but Torres shares the memories with a younger Enaran so they will not be lost.