A more thorough explanation:

Term: POSSLQ Definition: POSSLQ stands for "person of opposite sex sharing living quarters." It refers to someone who lives with a person of the opposite sex but is not married to them. For example, if a man and a woman live together but are not married, they could be considered POSSLQs. However, the term can also include married couples, which has caused some confusion. Another term, CUPOS, is more precise because it only refers to unmarried couples living together.