Often a term used among polyamorous (openly nonmonogamous) people. Often called the antonym of jealousy. Seeing your partner/s get a crush on, hook up with, fall in love with, or in any other way romantically bond with other people, and feeling deep happiness, joy, and love for them.
"You're in love with Parker, don't you feel jealous that he's started dating Emily?"
"Actually, no, I feel compersion. She's really cool. He lights up to see her. When I see him that happy, I actually feel happy for him, not jealous."