(Heck, some are involved in less than one.) Some people think the definition is a bit loose, but it's got to be fairly roomy to fit the wide range of poly arrangements out there.
In 1999, Morning Glory Zell-Ravenheart was asked by the editor of the Oxford English Dictionary to provide a definition of the term (which the dictionary had not previously recognised).
The two essential ingredients of the concept of polyamory are more than one; and loving.
That is, it is expected that the people in such relationships have a loving emotional bond, are involved in each other's lives multi-dimensionally, and care for each other.
Her definition was: The practice, state or ability of having more than one sexual loving relationship at the same time, with the full knowledge and consent of all partners involved.
This term was meant to be inclusive, and in that context, we have never intended to particularly exclude "Swinging" per se, if practitioners thereof wished to adopt the term and include themselves...
Polyamory is a hybrid word: poly is Greek for "many" (or "multiple") and amor is Latin for "love".
The article titled "A Bouquet of Lovers" written by Morning Glory Zell-Ravenheart and first published in Green Egg Magazine (Spring 1990), a publication founded by her husband Oberon Zell-Ravenheart, is widely cited as the original source of the word.
depends more on their attitude towards other "polyamorists", "swingers", etc., than on the exact nature of their relationships.
alt.polyamory participants collaborated on a FAQ (frequently asked questions) post that was updated periodically, and included the group's definition of "polyamory".
The latest version of the FAQ on polyamory.org, dated 1997, has this definition: Polyamory means "loving more than one".
This term is not intended to apply to merely casual recreational sex, anonymous orgies, one-night stands, pick-ups, prostitution, "cheating," serial monogamy, or the popular definition of swinging as "mate-swapping" parties.
that polyamory involves multiple consensual, loving relationships (or openness to such), but beyond that the term is as ambiguous as the word love itself.
A somewhat less common usage is to enumerate relationships by current sequential importance: the most important single relationship would be primary, the next most important would be secondary, the third would be tertiary, etc. Many polyamorous people however object to this sort of sequential "ranking".