Contubernium

As among the Romans, slaves had no civil state, their marriages, although valid according to natural law, when contracted with the consent of their masters, and when there was no legal bar to them, yet were without civil effects; they having none except what arose from natural law; a marriage of this kind was called contubernium. It was so called whether both or only one of the parties was a slave.
   

This entry contains material from Bouvier's Legal Dictionary, a work published in the 1850's.