Forms of nonmonogamy

Nonmonogamy is a blanket term covering several different types of interpersonal relationship in which some or all participants have multiple marital, sexual, and/or romantic partners. This can be contrasted with its opposite which is monogamy and yet may arise from the same Psychology of Monogamy.
Forms of nonmonogamy include:
consensual nonmonogamy, in which a nonmonogamous relationship is formed by mutual consent of those involved
casual relationship a physical and emotional relationship between two unmarried people who may have a sexual relationship
group marriage (also termed polygynandry), in which several people form a single family unit, with all considered to be married to one another
group sex and orgies involving more than two participants at the same time
infidelity, such as an affair, in which one participant fails to comply with expectations of a relationship.
Line families, a form of group marriage intended to outlive its original members by ongoing addition of new spouses
ménage à trois, a sexual (or sometimes domestic) arrangement involving three people
plural marriage, a form of polygyny associated with the The Church of Jesus Christ of Latter-day Saints in the 19th-century and with present-day splinter groups from that faith
polyamory, in which participants have multiple romantic partners
polyandry, in which women have multiple husbands
PolyFamilies, similar to group marriage, but some members may not consider themselves married to all other members
polyfidelity, in which participants have multiple partners but restrict sexual activity to within a certain group
polygamy, a term encompassing both polygyny and polyandry
polygyny, in which men have multiple wives
open marriage and open relationships, in which one or both members of a committed couple are sexually active with other partners
swinging, similar to open relationships, but commonly conducted as an organised social activity
As can be seen from this list, the Greek prefix 'poly-' (meaning 'many') is often used in naming nonmonogamous forms of relationship. In informal use such names are sometimes abbreviated to just 'poly', with the rest indicated by context.
Many nonmonogamous terms are flexible in definition, because they are based on criteria such as 'relationship' or 'love' that are themselves variably defined; see discussions at their respective pages for more detail on individual forms. In addition, usage creates distinctions beyond the raw definitions of the words. Thus, even though some relationships might technically be considered both polygamous and polyamorous, 'polygamy' usually signifies a codified form of multiple marriage, based on established religious teachings, while 'polyamory' is based on the preferences of the participants rather than social custom or established precedent.