Compersion

From Freepedia

Compersion is love manifested when a person takes joy in his or her partner's happiness with another person. It is a form of empathy; that is, pleasure that a loved one is experiencing a good thing in their life. It need not be sexual. This distinguishes it from erotic excitement in watching another person or persons behaving sexually, known as voyeurism. Feeling compersion for the first time has been described as frightening and disorienting because it is so different from normal concepts.

The term was coined by the Keristan Commune in San Francisco which practiced Polyfidelity, and is one of the core values of polyamory.

The synonym frubbly is widely used in the UK.