Hinge

In a closed polyamorous relationship where two people are NOT involved with eachother but are both involved with a third person, this third person is known as the hinge.

This comes from the visual of a hinge, with two things connected to it but not to eachother and still functioning as a whole.

This could be considered the opposite of a unicorn.

In Alex, Jaden, and Morgan's throuple, Alex is the hinge.

My crush turned out to be gay but he says he would totally share a hinge with me and I'm DOWN! Time to find us a man.