What is compersion? Feeling joy for a partner's happiness with someone else.
Compersion is the warm, genuinely happy feeling of seeing your partner happy with someone else — often described as the opposite of jealousy. It doesn't mean jealousy never shows up too; most poly people feel both at different times. Compersion isn't a requirement for doing non-monogamy "right," it's just a feeling some people notice once they're doing it.
