Polyamory (from Greek πολύ [poly, meaning many or several] and Latin amor [literally “love”]) is the desire, practice, or acceptance of having more than one loving, intimate relationship at a time with the full knowledge and consent of everyone involved. (Definition from Wikipedia).
In The Birthday Present the heroine is introduced to polyamory, and in the sequel The Christmas Present she and her husband explore polyamory further.
One of my favorite erotic romance authors recently wrote a blog post about polyamory. To read what Lisabet Sarai has to say, please check out Beyond Menage.