Poly relationship (from 'polygamy', 'polyamory' et al.) is a blanket term covering forms of interpersonal relationship in which some or all participants have multiple marital, sexual, and/or romantic partners. Such relationships are also sometimes termed nonmonogamous.

Forms of poly relationships, described in more detail at their respective pages, include:

• polygyny, in which men have multiple wives
• polyandry, in which women have multiple husbands
• polygamy, a term encompassing either of the above (but generally not both within the same relationship)
• plural marriage, a form of polygyny associated with the 19th-century Church of Jesus Christ of Latter-day Saints and with present-day splinter groups from that faith
• group marriage (also termed polygynandry), in which several people form a single family unit, with all considered to be married to one another
• Line families, a form of group marriage intended to outlive its original members by ongoing addition of new spouses
• PolyFamilies, similar to group marriage, but some members may not consider themselves married to all other members
• polyamory, in which participants have multiple romantic partners
• polyfidelity, in which participants have multiple partners but restrict sexual activity to within a certain group
• 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
• group sex and orgies involving more than two participants at the same time
• ménage à trois, a sexual (or sometimes domestic) arrangement involving three people

Many of these terms are flexible in definition, because the boundaries between sexual attraction and romantic love are not sharply defined. 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 established precedent.

The term 'poly relationship' is generally used only where all participants acknowledge the relationship as nonmonogamous. Thus, it is not applied where one person has multiple partners due to infidelity.

Sexual fidelity