Another new term, one that even a Google search didn't find in the right context. Cool! But are we talking about the same thing?

Normally I'd just use the term V-mate, for my lover's lover. However, since we're talking about my lovers' lover (note slight punctuation difference), that term wouldn't fully describe the relationship. Even though there are two Vs, W-mate isn't right either because that would mean my lover's lover's lover's lover, which in this set of people, no matter which way you follow the path, would be me. She's actually opposite me on a diamond, but diamond-mate sounds like a term somebody would use at the diamond distributor around the block from me.

So before I add "metamour" to my vocabulary, could you tell me the exact meaning? Thanks!