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 psychologist 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."