common law marriage

Definition from Nolo’s Plain-English Law Dictionary

In some states, a type of marriage in which couples can become legally married by living together for a long period of time, representing themselves as a married couple, and intending to be married. Contrary to popular belief, the couple must intend to be married and act as though they are for a common law marriage to take effect -- merely living together for a long time won't do it.

Definition provided by Nolo’s Plain-English Law Dictionary.