Joyousness is a feeling of being utterly delighted and happy. Your math class will erupt in joyousness at the news that a difficult exam is cancelled.
Joyousness is the state of experiencing joy, and we can trace the origin of both words back to the Latin root gaudia, "expressions of pleasure." Your joyousness often comes out when you're celebrating something wonderful, like your great-grandfather's birthday, or getting your driver's license, or the return of a long-lost friend. But there can also be a sense of joyousness during quieter moments of deep happiness.