This article is from the Polyamory FAQ, by email@example.com (H. Wilper) with numerous contributions by others.
Since there are lots of different ways to organize (or not
organize, if one is blessed by the Goddess of Chaos, or has a
taste for happy anarchy, or is a principled egalitarian)
relationships, it follows that there are ways of describing these
various arrangements. This polyjargon has evolved in the
newsgroup over time, and the words are merely descriptives. No
approval or disapproval of any particular arrangement is to be
expressed or implied.
Primary - word often used in a hierarchal multi-person
relationship to denote the person with whom one is most strongly
bonded. In some cases this bond or commitment takes the form of
legal marriage. As bigamy is not legal, the option of having two
(or more) legally wedded primaries simultaneously is not
currently practicable, though non-legal ceremonies may certainly
be performed. In some cases "primary" refers to the lover with
the most seniority.
Secondary - follows from primary, in a hierarchal relationship,
denotes a person with whom one is involved without the emotional,
legal, or economic complexities and commitments of primary
Yes, some people talk about tertiaries and so on. Some people
also don't like the terms primaries and secondaries or the
concepts behind the terms, preferring to have "a circle of
equals" as one poly person called it. Stef contributed the term
"Non-hierarchical Polyamory" for this kind of arrangement.
Triads - three people involved in some way. Often used in a
fairly committed sense, in some cases involving ceremonies of
commitment, but also used simply to mean "three people who are
connected". Example: "Jodine, Mischa and Mickey are a FMM triad
living in Excelsior."
Vee - Three people, where the structure puts one person at the
bottom, or "hinge" of the vee, also called the pivot point. In a
vee, the arm partners are not as commonly close to each other as
each is to the pivot.
Triangle (or equilateral triangle) - relationship where three
people are each involved with both of the others. Sometimes also
called a triad.
Line Marriage - term from the works of Robert A. Heinlein,
science fiction writer, meaning a marriage that from time to time
adds younger members, eventually establishing an equilibrium
population (spouses dying off at the same rate as new ones are
added). This is a different form of familial immortality than
the traditional one of successive generations of children.
(Definition courtesy of M. Schafer, and yes, there are people who
are in situations like this who use the term to describe their
Polyfidelity: Relationship involving more than two people who
have made a commitment to keep the sexual activity within the
group and not have outside partners. (Rumor has it that this
term was coined by the group Kerista.)
Quads, pentacles, sextets and more: There are polyfolk who exist
in multiple arrangements with more than three members. Geometry
can get complicated, and creative nomenclature abounds. As in
every other aspect of polyamory, the precise bonds of intimacy
vary from group to group and from member to member within groups.