compersion

A major component of jealousy, for many people, is the suspicion or resentment of the rival. In Kerista, on the other hand, the relationship that every man had with the other men and that every woman had with the other women elicited what they called "compersion" rather than jealousy. "Compersion is the positive warm feeling you experience when you see two of your partners having fun with each other. It's the antonym of jealousy."

1992, Ayala Malach Pines, “Romantic Jealousy in Open Relationships”, in Romantic Jealousy: Causes, Symptoms, Cures, New York, N.Y.; London: Routledge, published 1998, →ISBN, page 140

Some polyamorous people experience compersion, which means feeling joy that one’s partner is sharing closeness with another person[…]. [M. C.] Keener's (2004) study participants noted an absence of worry as to whether their partners might be cheating, since the permission to take other lovers was granted freely.

2006, Geri Weitzman, “Therapy with Clients who are Bisexual and Polyamorous”, in Ronald C. Fox, editor, Affirmative Psychotherapy with Bisexual Women and Bisexual Men, New York, N.Y.; Hove, East Sussex: Routledge, published 2013, →ISBN, page 140

One last quality that cannot go unmentioned in any discussion of open relationships is compersion, a concept that may be new to many readers. Compersion is taking joy in your partner's pleasure or happiness with another partner. […] Some practitioners of polyamory think of compersion as the opposite of jealousy, or at least the antidote to jealousy. Given the problems (and drama) ignited by jealousy, you can see how compersion can go a long way toward creating a foundation for pleasure and generosity in any relationship.

2008, Tristan Taormino, “What Makes an Open Relationship Work?”, in Opening Up: A Guide to Creating and Sustaining Open Relationships, San Francisco, Calif.: Cleis Press, →ISBN, section 1 (Choosing an Open Relationship), page 49