Polyamory (not to be confused with polysexuality) is a style or philosophy toward relationships that recognizes that an individual can ethically be involved in more than one sexual or romantic relationship at any given time, as opposed to the socially normative convention of monogamy.
Polyamory is a form of ethical non-monogamy, an umbrella term that encapsulates activities such as swinging and kink, and philosophies such as free love.
In the 1980s and 1990s, the initial luster of swinging and partner-swapping experienced by many couples gave way to a desire to have more emotionally fulfilling and longer lasting encounters outside of the coupling.
In 1990, the term "polyamorous" was coined to mean "having many loves", and in 1992 the word "polyamory" was created in order to accommodate the Usenet group on the subject, alt.polyamory.
The idea that a relationship must travel upward in intensity (e.g., meeting to dating to moving in together to getting married to having kids to growing old and retiring together) is fairly antithetical to polyamory—in polyamory, the "escalator" can move backwards, stop indefinitely at certain floors, and different people can get on and off without being a disaster.
Also thrown in the trash is the conventional idea of a "successful relationship", which Dan Savage has described as one which lasts until one of the partner dies.
Recognizing this complexity actually explains why these arrangements can actually be difficult to maintain, but it is also that recognition that facilitates the arrangement and addressing all the issues that can occur within it.
A concept of contention even within the community is the "relationship hierarchy", in where there exists a core or primary relationship (usually a partnered couple, possibly married) and all other relationships are secondary, tertiary or otherwise subordinate to the primary relationship.
Because of the spectra of gender identities, sexualities, physiologies and just general attitudes of individuals, there are multiple forms of polyamory.
In 2010, Christopher Ryan and Calcida Jetha published Sex at Dawn which examined the history and biology of non-monogamy.
In 2011, The Ethical Slut hit its second edition with new information and stories.
A successful relationship for the polyamorous is one which enriches the lives of all involved in the relationship for however long the relationship lasts.
In the first and second waves, the concept of a "relationship" was regarded as being all the members involved.