Unicorn- A bisexual woman who has sex with hererosexual married couples
So-So- Your significant other's significant other.
Throuple- A couple plus one
Solo Poly-having more than one partner but no "main parter"
Anchor partner- The person you've been with longest and may live with.
Open relationship - A committed relationship between two partners who also sleep with other people but may have to get permission in advance.
Polyamory- The practice of maintaining intimate relationships with multiple committed partners.