Non-monogamy (or nonmonogamy) is an umbrella term for every practice or philosophy of non-dyadic intimate relationship that does not strictly hew to the standards of monogamy, particularly that of having only one person with whom to exchange sex, love, and/or affection. In that sense, "nonmonogamy" may be accurately applied to extramarital sex, group marriage, or polyamory. It is not synonymous with infidelity, since all parties are consenting to the relationship structure, partners are often committed to each other as well as to their other partners and cheating is still considered problematic behavior with many non-monogamous relationships.
More specifically, "nonmonogamy" indicates forms of interpersonal relationship, intentionally undertaken, in which demands for exclusivity (of sexual interaction or emotional connection, for example) are attenuated or eliminated, and individuals may form multiple and simultaneous sexual and/or romantic bonds. This stands in contrast to monogamy, yet may arise from the same psychology. According to Jessica Fern, a psychotherapist and the author of Polysecure: Attachment, Trauma and Consensual Nonmonogamy, as of September 2020, about 4% of Americans, nearly 16 million people, are "practising a non-monogamous style of relationship" while the a 2016 study said that over 21% of Americans engaged in consensual non-monogamy at "some point in their lifetime." In January 2020, a YouGov poll found that about one-third of US adults believe that "their ideal relationship is non-monogamous to some degree."
Terminology
Many terms for non-monogamous practices are vague, being based on criteria such as "relationship" or "love" that are themselves subjectively defined. There are forms of non-monogamy whose practitioners set themselves apart by qualifiers, such as "ethically non-monogamous" which intends a distancing from the deceit or subterfuge they perceive in common cheating and adultery. This usage creates distinctions beyond the definitions of the words. For example, though some relations might literally be both polygamous and polyamorous, polygamy usually signifies a codified form of multiple marriage, based on established religious teachings such as Plural marriage, a form of polygyny associated with the Latter Day Saint movement in the 19th-century and with present-day splinter groups from that faith, as well as evangelical sects that advocate Christian Plural Marriage.
Polyamory is based on the preferences of the participants rather than social custom or established precedent. There is no one 'right' way to engage in non-monogamy (although there are widely agreed on 'wrong' ways). Because of this, the terms for the various kinds of relationships can be vague and sometimes interchangeable. but there are some distinctions that are worth defining. For example, swingers may intentionally avoid emotional and social connection to those—other than their primary partner—with whom they have sex, so may or may not be polyamorous but are non-monogamous.
Some useful terms are Metamour or Meta, the common term for a person with whom a partner is shared, V-Structure, one person is equally involved with two partners, and Triads / Quads. The latter is when three or four participants make up the primary partnership.