May 16, 2014 update: a MUCH shorter (and, for me, more life-accurate) version of this post can be found at Definitions, Redux.
I will preface this by saying these definitions are my own, distilled from reading and personal experience. There is no open relationship académie française; no one gets to declare canonical definitions. So if you run across someone who disagrees with me, have a dialogue and decide which concepts work for you.
I am nonmonogamous, and my primary is polyamorous.
I date people in clutches, seeing each guy individually once or twice a month with periods of radio silence in between. Beyond practicing safer sex, we have no obligations to or claims upon one another. We are affectionate and supportive, but none of these pairings are "relationships."
This is my nonmonogamy: I sleep with friends.
Dodi is polyamorous. Dodi loves. He loves widely, passionately, and sometimes intimately. He is currently dating me and one other woman. He thinks of and refers to both of us as his girlfriends, though we've agreed that I am his primary and he is mine.
Dodi behaves as all the best boyfriends do: he keeps himself apprised of our lives. He nurtures our physical and emotional intimacies. He makes small and large gestures of affection, making sure we both know without question how he feels about us.
This is his polyamory: he loves two women deeply and happily.
Within my experience, what we have is a little odd; usually, both partners have the same relationship style. Dodi and I are the Owl and the Pussycat, two species dating with crossed fingers and a lot of hope. There may come a time when he chooses to date more casually, or I find a secondary , but for the moment, these are our working definitions.
As a semi-aside...you may hear those unfamiliar with the shadings of polyamory and nonmonogamy use the term "swinger" to describe any open relationship. Personally, I find this reductive and unintentionally offensive. Here's why:
Swinging is usually -not always, but usually- about no-strings sex. Sometimes it's a one-off, sometimes it's with strangers, sometimes it's a blending of both of these with other stuff thrown in, or not. Though many couples swing together, the resulting...encounters...tend to be about sex and unrelated to friendships or relationships.
Ain't nothing wrong with any of this. In my opinion, so long as you play safe-r, the boundaries of your interactions are none of my damn business. But swinging is not what Ido.
I sleep with friends. I'll go on more than a few dates with someone before I sleep with him. Names, emails and phone numbers are exchanged. Long before clothing disappears, we'll have A Grown-Up Conversation and confirm that we are willing to take the risks inherent to any sexual interaction. We are not groping in a dark corner and slinking away.
Not that that's what swingers do.
I mean, some do. Not that there's anything wrong with that.
More than once, I've explained the above to someone (though in person, and with many fewer compound sentences), and gotten a blank stare. The differences are obvious to me, but still blurry to the listener.
So here's the litmus test: if you would make a Major Life Decision without factoring its impact upon a given partner, that relationship is nonmonogamous rather than polyamorous.
Boom. Done. : )