I find the discussion of the word "metamour" interesting. I, personally, like it but think it's really only useful short-hand for discussions with other poly-aware people - for the general population I would use the long hand, "my husband's girlfriend" for instance.
We don't tend to use the term to describe the relationship between MrS and Dude because their OTHER relationships to each other - best friends, roommates - are actually more important and intimate than is implied, to me, by the concept. I would be more likely to apply it to someone to which I had NO other relationship - say someone that Dude was seriously dating whom I had yet to meet. She would be peripheral to my life but have an effect, through him, on it.
(An aside, I find it interesting to note that different languages have both more and less specific terms to define certain relationships depending on the culture. For instance "sister-in-law" could mean my husband's sister or my brother's wife. Some languages have different words for "paternal" vs. "maternal" aunt and whether they are the blood relative or not - whereas in English my mother's sister and my father's brother's wife are both my "aunt")
Me: poly bi female, in an "open-but-not-looking" V-plus with -
MrS: hetero polyflexible male, live-in husband (24+ yrs)
Dude: hetero poly male, live-in boyfriend (5+ yrs) and MrS's BFF
SLeW: platonic girlfriend and BFF
Lotus: "it's complicated"
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe