Just as there are many different types of poly people, so are there many definitions of the term polyamory. There is not only polaymory but many polyamories. (Just as there are many monogamies: many different ways to enact monogamy.) Below is my current provisional definition of polyamory. It’s open to revision, of course. I welcome comment/critique, as I work to make this definition better and better. Thanks!
Polyamory is a relationship orientation and a cultural worldwide movement that rejects the assumption that love is a finite resource. Thus, to be polyamorous is to believe that abundant love is possible: within spiritual, sexual, emotional, and/or intellectual relationships between honest, communicative, consenting adults. As an orientation toward the world that is an alternative to monogamy (but not necessarily “against” monogamy–for monogamy may work for some people), a key tenant is the notion that it is possible to ethically and responsibly love more than one person at a time. However, a polyamorous person may identify as such no matter what form their various relationships take. In other words, one does not have to have multiple partners to be identified as poly (e.g an unattached single person can be polyamorous).
Other closely related terms but not quite synonymous terms are are “ethically non-monogomous” and “open.”