What is "syneisaktism"

Syneisaktism is the practice of "spiritual marriage", which is where a man and a woman who have both taken vows of chastity live together in a chaste and non-legalized partnership. More often than not, the woman would move into the house of the man, and they would live as brother and sister, both committed to the continuation of their vows of chastity. The women who entered into a spiritual marriage were known as subintroductae ("those brought in covertly"), agapetae ("beloved ones"), and syneisaktoi ("those brought into the house together"). This practice emerged around the 2nd century CE, and survived into the Middle Ages, despite being condemned by numerous church leaders, writers, and councils.