Happiness is an emotion associated with feelings ranging from contentment and satisfaction to bliss and intense joy. A variety of philosophical, religious, psychological and biological approaches have been taken to defining happiness and identifying its sources.

Philosophers and religious thinkers have often defined happiness in terms of living a good life, or flourishing, rather than simply as an emotion. Happiness in this older sense was used to translate the Greek Eudaimonia, and is still used in virtue ethics. In everyday speech today, however, terms such as well-being or quality of life are usually used to signify the classical meaning, and happiness is reserved for the felt experience or experiences that philosophers historically called pleasure.

Happiness forms a central theme of Buddhist teachings, which focuses on obtaining freedom from suffering by following the Eightfold Path. In the Buddhist view, ultimate happiness is only achieved by overcoming craving in all forms. Aristotle saw happiness as "the virtuous activity of the soul in accordance with reason," or the practice of virtue. In Catholicism, the ultimate end of human existence consists in felicity, or "blessed happiness", described by the 13th-century philosopher-theologian Thomas Aquinas as a Beatific Vision of God's essence in the next life. One psychological approach, positive psychology, describes happiness as consisting of positive emotions and positive activities. In most religions, happiness is the eternal reward for those who meet certain criteria.

While direct measurement of happiness is difficult, tools such as The Oxford Happiness Inventory have been developed by researchers. Physiological correlates to happiness can be measured through a variety of techniques, and survey research can be based on self-reported happiness levels.

Research has identified a number of correlates with happiness. These include religious involvement, parenthood, marital status, age and income. Happiness economics suggests that measures of public happiness should be used to supplement more traditional economic measures when evaluating the success of public policy.

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