The warm, happy feeling you get from seeing a partner enjoy another relationship — often called the opposite of jealousy.
If jealousy is the knot in your stomach when your partner lights up for someone else, compersion is the opposite reflex: you see them glowing, and it makes you glad. Not gritted-teeth tolerance — actual warmth. Polyamorous communities often frame it as jealousy's antidote, the feeling that makes sharing a partner feel like abundance rather than loss.
The big misconception is that compersion is mandatory — that a "good" non-monogamous person feels it automatically, and jealousy means you're doing it wrong. That's not how the people who live it describe it. Compersion is a capacity, not a virtue test; it often shows up in flashes, coexists with insecurity, and is something you grow toward rather than switch on. Plenty of experienced practitioners are blunt that it isn't guaranteed and shouldn't be faked.
In practice it's less a fireworks moment and more a quiet recalibration: your partner comes home buzzing from a good date and you catch yourself smiling instead of bracing. It doesn't erase jealousy — the two can share the same evening. The word matters precisely because English never had one for this, which tells you something about the assumptions baked into the language we inherited.