Cohabitation refers to a living arrangement in which two people in a relationship live together on a long-term basis without being married.
Cohabitation is sometimes also referred to as a live-in relationship.
Cohabitation is not a new concept although it has become widely prevalent in the last few decades. Prior to the 1970s, it was mostly illegal in the United States for unmarried couples to live together due to various law against fornication, cohabitation, and adultery. Now, cohabitation is not considered illegal. Some states, including Idaho, Illinois, and South Carolina still have laws against fornication between unmarried individuals. These are rarely, if ever, enforced and have been deemed unconstitutional. However, the law has always been in favor of marriage, owing to the general perception that marriage is important for the stability of a family. That is why cohabiting couples aren't given as many rights or privileges as their married counterparts.