Compersion is often defined as "the opposite of jealousy." It is an empathetic state of happiness and joy spurred by another individual's experience of happiness and joy. The term was coined by the polyamory community. It is often employed in the context of polyamorous relationships to describe the positive feelings experienced by a person when their lover is enjoying another relationship.
More About Compersion
Compersion is embraced in different ways within the polyamory community. Some use it to describe the fact that their partner's joy serves as a turn on for them, while others use it more simply to describe the joy they take in their partner's happiness. Others see compersion as the ability to turn jealousy into acceptance. In all of these scenarios compersion is the key to maintaining harmony in non-monogamous relationships.