Polyamory means "loving more than one". This love may be sexual, emotional, spiritual, or any combination thereof, according to the desires and agreements of the individuals involved, but you needn't wear yourself out trying to figure out ways to fit fondness for apple pie, or filial piety, or a passion for the Saint Paul Saints baseball club into it. "Polyamorous" is also used as a descriptive term by people who are open to more than one relationship even if they are not currently involved in more than one. (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.
But isn't that "cheating"?
Oh, you wanted a longer answer. Okay.
According to the OED,cheating means "fraud, deceit, swindling." There's a nice quote from 1532: "The first...ground of Chetinge is...a studdy to seme to be, and not to be in deede." In other words, cheating is to convey through deliberate action the impression that one is of a particular nature while one is, in fact, something quite different. What this boils down to with polyamory is that polyamorous people do not tell partners, lovers, or prospective members of those groups that they are monogamous when in fact they are not -- nor do they allow these people to assume they are monogamous, regardless of how convenient or personally advantageous such assumptions might be. The words "honest", "negotiate", "communication" and "being out" occur frequently in discussions of how polyamory usually works.
As Stef puts it:
"I think the key in defining polyamory is *openness*, that is, having multiple relationships with the knowledge and consent of your partner(s) rather than by deceit. (How much openness, how many details are shared, of course varies widely.) A great many people have secret affairs while they're in a supposedly monogamous relationship. I think those people might have the potential to be polyamorous, but I do not think they are practicing polyamory. Another key in defining polyamory, IMO, is that it need not involve sex (although it often does)."
Generally speaking, if someone openly practices "more than one love" and calls themself polyamorous, they probably are; if they practice "more than one love" and call themself monogamous, do not adjust your television: the problem is *not* in your receiver.