This is an attempt to give you a brief overview of the most important words for poly-friendly people.
Also, some important definitions can be found here:
(Note: As with most of the rest of the English language, precise definitions of these words vary from person to person and from group to group. Your mileage may vary.)
Closed – A relationship is closed if there is an agreement among the members to not get sexually and/or romantically involved with anyone outside the relationship. A monogamous relationship is a closed two-person relationship.
Group Marriage – A marriage involving more than two people. Not legally recognized in the U.S. or by most major religions in western society. Many people do it anyway, often making legal agreements that function similarly to the legalities in a usual marriage.
Open – A relationship is open if there is an agreement among the members that it is acceptable to get sexually and/or romantically involved outside of the relationship. Specific rules within a relationship about such involvement are usually negotiated to best fit the people in the relationship. See the section on “Agreements”.
Partner – Short for “life partner.” Gender-free, hetero-assumption-free term for someone with whom one is involved, usually in a primary relationship. Also sometimes used in different contexts as short for “sexual partner”.
Poly – Short for polyamorous, usually used as an adjective. A poly relationship is a non-monogamous relationship. A poly person is someone in, or at least interested in, a poly relationship. See polyamory.
Polyamory – Literally, “many loves.” A generic term for non-monogamy.
Polyandry – Having more than one husband.
Polyfidelity – Closed relationship involving more than two people. The members of a group marriage, for example, may limit their sexual/romantic involvement to members of the group.
Polygamy – Having more than one spouse.
Polygyny – Having more than one wife.
Primary Relationship(s) – The relationship(s) which is (are) the most important and typically involve a high degree of commitment, such as the relationship with a marriage partner.
Safe Sex – Also known as safer sex. Ways of being sexual while trying to minimize the odds of contracting STDs (sexually transmitted diseases), particularly AIDS. This is done by minimizing the exchange of bodily fluids (especially semen, vaginal secretions, and blood), usually by using latex barriers such as condoms and dental dams for intercourse and oral sex, and/or by limiting sexual practices to less risky ones (e.g. mutual masturbation).
Secondary Relationship(s) – Close, ongoing emotional and/or sexual relationship(s), but with a lesser degree of commitment than a primary relationship.
Sex Negative – An word often used to describe people who think of sexuality in general, or any sexuality different from their own as dirty or sinful.
Sex Positive – An word often used to describe people who are comfortable with their own sexuality and sexuality in general.
Swinging – A sub-culture of people who enjoy being sexual with acquaintances and/or friends. (This word seems to be a point of some contention. Some people associate “swinging” with sexist, indiscriminate, dishonest, and/or risky sexual behavior. Probably, behavior like this does or doesn’t happen, unrelated to what the participants call themselves.)
Tertiary Relationship(s) – Emotional and/or sexual relationship(s) with little or no ongoing involvement.
Triad – A relationship involving three people.