Mudita
Muditā (Pāli and Sanskrit: मुदिता) is a dharmic concept of joy, particularly an especially sympathetic or vicarious joy—the pleasure that comes from delighting in other people's well-being.The traditional paradigmatic example of this mind-state is the attitude of a parent observing a growing child's accomplishments and successes. Mudita should not be confused with pride, as a person feeling mudita may not have any benefit or direct income from the accomplishments of the other. Mudita is a pure joy unadulterated by self-interest.

Application
Mudita meditation cultivates appreciative joy at the success and good fortune of others. The Buddha described this variety of meditation in this way:

Here, O, Monks, a disciple lets his mind pervade one quarter of the world with thoughts of unselfish joy, and so the second, and so the third, and so the fourth. And thus the whole wide world, above, below, around, everywhere and equally, he continues to pervade with a heart of unselfish joy, abundant, grown great, measureless, without hostility or ill-will.
Buddhist teachers compare mudita to an inner spring of infinite joy that is available to everyone at all times, regardless of circumstances.

Joy is also traditionally regarded as the most difficult to cultivate of the four immeasurables (brahmavihārā: also "four sublime attitudes"). To show joy is to celebrate happiness and achievement in others even when we are facing tragedy ourselves.According to Buddhist teacher Ayya Khema showing joy towards sadistic pleasure is wrong. Here there should instead be compassion (karuṇā).
The "far enemies" of joy are jealousy (envy) and greed, mind-states in obvious opposition. Joy's "near enemy," the quality which superficially resembles joy but is in fact more subtly in opposition to it, is exhilaration, described as a grasping at pleasant experience out of a sense of insufficiency or lack.

See also
Brahmavihara
Karuṇā (compassion)
Metta (loving-kindness)
Mind Stream
Pīti (joy)
Sukha (happiness)
Upekkha (equanimity)
Similar concepts in other cultures:
Naches—A Yiddish term with a very similar meaning
Firgun—A Hebrew term with a similar meaning
Schadenfreude—A German word meaning to take joy in another's misfortune

References
External links
Four Sublime States, and, The Practice of Loving Kindness (PDF). The Wheel. Vol. 6/7. Translated by Ñāṇamoli, Bhikkhu; Nyanaponika, Thera. Kandy, Sri Lanka: Buddhist Publication Society. 2008 [1958]. ISBN 955-24-0170-4.
Amaro, Ajahn (2016-07-29). "Just One More…". Amaravati.
Napier, DaeJa. "Mudita: Appreciative Joy".