Happiness is a mental or emotional state of well-being which can be defined by, among others, positive or pleasant emotions ranging from contentment to intense joy. Happy mental states may also reflect judgements by a person about their overall well-being. A variety of biological, psychological, economic, religious and philosophical approaches have striven to define happiness and identify its sources. Various research programmes, including positive psychology and happiness economics, are employing the scientific method to research questions about what "happiness" is and how it might be attained.
The United Nations declared 20 March the International Day of Happiness to recognise the relevance of happiness and well-being as universal goals.
Philosophers and religious thinkers often define happiness in terms of living a good life, or flourishing, rather than simply as an emotion. Happiness in this sense was used to translate the Greek eudaimonia, and is still used in virtue ethics. There has been a transition over time from emphasis on the happiness of virtue to the virtue of happiness. Since the turn of the millennium, the human flourishing approach, advanced particularly by Amartya Sen has attracted increasing interest in psychological, especially prominent in the work of Martin Seligman, Ed Diener and Ruut Veenhoven, and international development and medical research in the work of Paul Anand.
A widely discussed political value expressed in the United States Declaration of Independence of 1776, written by Thomas Jefferson, is the universal right to "the pursuit of happiness." This seems to suggest a subjective interpretation but one that nonetheless goes beyond emotions alone. In fact, this discussion is often based on the naive assumption that the word happiness meant the same thing in 1776 as it does today. In fact, happiness meant "prosperity, thriving, wellbeing" in the 18th century.