person of the opposite sex sharing living quarters
This is a rather useless term that the US Census Bureau tried to use for “significant others” — people who aren't necessarily married, but are living together. However, you'll note that it excludes gay and lesbian couples, which is why it sucks.
A possibly even more obscure word than “POSSLQ” (for “person of the opposite sex sharing living quarters”) which an former roommate of mine used to refer to our living arrangements (namely, men and women, living together chastely--at least with each other!) was syneisaktism (related obscure term: subintroducta), which, for instance, doesn't even appear in the OED (the Oxford English Dictionary, the most complete English dictionary around). It actually means more than just “chaste roomies of the opposite sex,” because it really refers to spiritual marriage of people of the opposite sex, a pure marriage of two spiritual souls who therefore do not engage in sexual relations, and in effect, eliminate their gender. This was the very thing that ticked off people like John Chrysostom about syneisaktism: the elimination of gender meant the elimination of male superiority (oh-oh!).
See these links:
A fragment of a review article: “This fascinating book, the outgrowth of a doctoral dissertation at Duke University, has its inception in two treatises of John Chrysostom (ca. 347-407 C.E.) against spiritual marriage (syneisaktism), the practice of heterosexuals living together in an allegedly nonsexual manner for ascetic purposes” can be found here:
Review of Theatrical Shows and Ascetic Lives: John Chrysostom's Attack on Spiritual Marriage by Blake Leyerle (Berkeley: University of California Press, 2001).
If you can access articles on JSTOR, or have access to a good library, then look up this one: “John Chrysostom and the Subintroductae” by Elizabeth Clark [Church History, Vol. 46, No. 2. (Jun., 1977), pp. 171-185]--it... [More...]