living apart together
n. A situation in which an unmarried couple live in separate residences while maintaining an intimate relationship; a person in such a relationship.
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Another twist on the traditional family is that one out of every 12 Canadians was living apart from a partner in 2001 — most of them young adults. Many were living with their parents, the report found. In all, eight per cent of the population aged 20 and over were part of what is being termed LAT, Living Apart Together, relationships, said Statistics Canada.

While most of those living apart from a partner — 56 per cent — were in their 20s, 19 per cent were in their 30s, 14 per cent in their 40s and 11 per cent were 50 and over. The survey also found that 36 per cent of those living apart lived with their parents.

Many Canadians involved in such relationships see the arrangement as a precursor to marriage; for others careers mean this type of union may be more permanent. About one-half of Living Apart Together couples expect to live common law in the future.
—“Fewer want traditional family: Statistics Canada,” The Record (Kitchener-Waterloo, Ontario), June 11, 2003
My bloke and I already exist happily as LATs (Living Apart Together) and there are compelling environmental reasons for our decision. Noise pollution, for one. Our arguments are the sort of things that keep decibel-measuring men from the council employed full-time. Also, we both know my way of life would damage my bloke's mental environment. When he discovered I keep my screwdriver kit in an Alannah Hill velvet purse, fear showed in his eyes and talk of cohabiting ended. I think the poor man imagined himself arranged in the corner, draped in a beaded throw.
—Amy Cooper, “Changing Places,” Sun Herald (Sydney), March 09, 2003
1989 (earliest)
Frederick Barthelme is adept at writing short stories — until he tries to turn one into a novel. But that is precisely what he has done with his fifth book, Two Against One, his portrait of a modern marriage teetering on the brink of relapse. It tells the story of one weekend in the lives of Edward and Elise, a separated couple considering such yuppie marriage remedies as living apart together and living together with a third.
—Jason Sherman, “Inside this flabby novel lurks a taut short story,” The Toronto Star, March 11, 1989
Artois Piedboeuf Interbrew was formed two years ago to bring together a large number of chiefly export-oriented activities. Since then the two sides have grown closer together, and they now enjoy what De Financieel Ekonomische Tijd terms a "living apart together" relationship.
—“Artois Piedboeuf Interbrew has announced that it has effected certain organisational changes in order to improve efficiency ahead of the creation of the single European market in 1993,” Textline Multiple Source Collection, January 14, 1988