Compersion
Vicarious joy, experiencing pleasure because of another person’s pleasure. Basically the opposite of jealousy.
Coined by the Kerista Community in the 1970s, probably following the model of dispersion, so that use specifically applies to polyamorous relationships, the pleasure of your partner experiencing pleasure with someone else.1
Not unlike the Buddhist concept of sympathetic joy (mudita in Sanskrit). Or Neitzsche coining mitfreude.