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.