Compersion is typically defined in the polyamorous community as happiness at the happiness of one's partner with their other partner(s). Some people go so far as to use the phrase "the opposite of jealousy." Laura discusses the concept, the possibility of broadening this definition to "positive empathy for one's partner" and whether that waters down the concept too far (and if so, if we need the concept) - with Kimota Tigerlily and Seva.
The episode can be found at http://readyforpolyamory.fireside.fm/compersion