Elsewhere on this website, we said that in order to be happy we need two things: positive emotions and a sense that our life has a purpose.
While this is a good starting point, this definition is just one of many. For a brief (critical) overview of contemporary theories of happiness see Martin E.P. Seligman and Ed Royzman’s “The Three Traditional Theories/Authentic Happiness,” at http://www.authentichappiness.sas.upenn.edu.
For historical perspectives on happiness, visit https://www.pursuit-of-happiness.org/
Darrin McMahon’s, Happiness: A History, is another good source.