The term mudita is usually translated as "sympathetic" or "altruistic" joy, the pleasure that comes from delighting in other people's well-being rather than begrudging it. Many Buddhist teachers interpret mudita more broadly as referring to an inner spring of infinite joy that is available to everyone at all times, regardless of circumstances. The more deeply one drinks of this spring, the more secure one becomes in one's own abundant happiness, and the easier it then becomes to relish the joy of other people as well.
The traditional example of the mind-state of mudita is the attitude of a parent observing a growing child's accomplishments and successes.
Mudita is also traditionally regarded as the most difficult of the brahmaviharas to cultivate. To show mudita is to celebrate happiness and achievement in others even when we are facing tragedy ourselves.
The "far enemies" of mudita are jealousy and envy, two mind-states in obvious opposition. Mudita's "near enemy," or quality which superficially resembles mudita but is in fact more subtly in opposition to it, is exhilaration, perceived as a grasping at pleasant experience out of a sense of insufficiency or lack.
Schadenfreude, a german word also used in English, is the exact antonym of Mudita. Schadenfreude is enjoyment taken from the misfortune of someone else.