Once upon a time if you referred to someone as your "partner" most people would have assumed you meant your business partner. But these days "partner" is widely used to refer to romantic partners or significant others.
I'd say this usage has been around for at least 20 years. In the lead-up to our wedding in 1993 I remember referring to Kevin as my "partner" since he'd been promoted from a mere "boyfriend" and I couldn't bring myself to utter the cloyingly prissy - yet strictly accurate - "fiancé".
I suspect we New Zealanders were ahead of the rest of the world in adopting "partner" in this fashion. A 2009 Columbia News Service story entitled Take my partner, please! Couples embrace new term for each other gave the impression this was a fairly recent phenomenon in the US. It explained that "[i]n an age of increasingly diverse and complicated relationships, couples are searching for the right term to call their one-and-only. Many are turning to the term partner to describe everything from a wife to a casual boyfriend."
The appeal of "partner" lies in its gender neutrality as well as the fact that it is no nonsense and unsentimental - qualities that appeal to uncomplicated Kiwis. Yet some people lament the fact that the new meaning of this word has gained such widespread acceptance and wish there was an alternative word for referring to someone in a committed relationship that doesn't involve marriage.
But what other terms are suitable for this purpose? Let's take a look at some possibilities.