Consensual non-monogamy (CNM) is blanket term that refers to relationships that are ethically open. Examples of consensual non-monogamy include, among others, polyamory and swinging.
Consensual non-monogamy is marked by open, honest communication among all partners as well as informed consent -- everyone knows exactly what they are getting into and no one is hiding anything.
More About Consensual Non-Monogamy
Consensual non-monogamy allows participants to have multiple partners and/or relationships without conventional "cheating." That said, successful consensual non-monogamy often involves structure and well-defined boundaries.
The term "ethical non-monogamy" can also be used to refer to the kinds of relationships that fall under the umbrella of consensual non-monogamy.