Which term refers to the Buddhist concept of joy in the happiness of others?

The term that refers to the Buddhist concept of joy in the happiness of others is known as Mudita. This is one of the four sublime states in Buddhism, often referred to in the context of developing loving-kindness and compassion. Mudita specifically represents a sense of vicarious joy and sympathetic joy, where an individual feels genuine happiness for the well-being and achievements of others, rather than being envious or resentful.

This concept emphasizes altruism and the ability to rejoice in the successes of others, promoting a sense of community and interconnectedness which is essential in Buddhist teachings. It contrasts with negative emotions like jealousy or resentment, illustrating an important aspect of an enlightened mind that celebrates the happiness and prosperity of others as much as one's own.

Other choices represent different concepts in Buddhism: Karuna refers to compassion and the desire to alleviate suffering, Upekkha denotes equanimity or a balanced mental state amidst joy and sorrow, and Metta signifies loving-kindness and unconditional love towards oneself and others. Each of these plays a role in the cultivation of a positive and wholesome mind but does not specifically capture the joyous aspect of rejoicing in others' happiness as Mudita does.