vee

A convergence of water caused by a narrowing of the river. Often throws up standing waves at the downstream end. Can create good surf waves as well.

Polyamory definition is:
A relationship in which one person is the point of the vee, with two people to whom s/he is closely bonded. The two other people are not involved with each other. In that case, it becomes a triad.

For example:
Y is closely involved with both X and Z.
X and Z likely know each other/are friends etc. but are not involved with each other - only with Y.