Were I to taboo "polyamory", I would unpack it as the practice of maintaining romantic relationships with more than one person at a time.

With the caveat that every partner knows about it and consented to the arrangement, and not under duress. Otherwise it would be called cheating and/or coercion, which is far more widespread than polyamory. On a related note, when comparing polyamory to other arrangements, one has to account for the effects of cheating in a monogamous arrangement.