"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness."  -- Thomas Jefferson, Declaration of Independence. July 4, 1776

It's most likely Jefferson stole the phrase "the pursuit of Happiness" from Locke's An Essay Concerning Human Understanding: The necessity of pursuing happiness [is] the foundation of liberty.  As therefore the highest perfection of intellectual nature lies in a careful and constant pursuit of true and solid happiness; so the care of ourselves, that we mistake not imaginary for real happiness, is the necessary foundation of our liberty. The stronger ties we have to an unalterable pursuit of happiness in general, which is our greatest good, and which, as such, our desires always follow, the more are we free from any necessary determination of our will to any particular action…”

(Jefferson is also clearly influenced by Locke's phrase "Civil interests I call life, liberty, health, and indolency of body; and the possession of outward things, such as money, lands, houses, furniture, and the like," from Locke's "Letter On Toleration" (from which we also gain our concept of the separation of church and state ("civil interests")).

Eudaimonia: the Greek word for “happiness”, or more accurately, “the flourishing life” or “the good life”.  But we are not referring to an emotional state or pleasure, but rather a fulfilled life, one that is lived in accord to our deepest values and aspirations not just for ourselves but for our families and community.  In the Nicomachean Ethics, Aristotle writes, “the happy man lives well and does well; for we have practically defined happiness as a sort of good life and good action.” 

For Aristotle and Epicurus Eudaimonia is derived from arête: "virtue" or “excellence”: one being at one’s best.  Usually it also contains a civil element: we are measured in relation to our families, communities, friends.   In other words, one attains virtue usually by service or sacrifice to others. 

This seems similar to the Hebrew scriptures use of the word ahav or ahev to mean "love", the root of which is "give" and is used in context of God's covenant: a political and/or sacred contract.  For both the Classical Greeks and ancient Hebrews, one achieves "happiness" through virtuous treatment of others.  For the Hebrews this is emulating God; for the Classical Greeks, it's simply rational.

Starting with Socrates, the Greek philosophers were really mainly interested in this: the ability to judge what actions helped to cultivate our sense of eudaimonia Every question that Socrates asked looped back to that central set of questions:   what are the values upon which you have based your life -- your beliefs, your choices, and your actions -- and what are the assumptions necessary to support those values.

What Socrates brought to Athens, and so to the world, was the idea that we should pester each other thinking about these questions because we must make choices that effect one another: my vision of eudaimonia doesn’t just effect me and my family, but you and your family, and of course yours effects mine.

The Socratic method of teaching was simple: one simply asks another to a series of questions about our basic assumptions and forces that person to identify those assumptions and see whether or not it is possible to support them.

So, when Jefferson uses the term "happiness", he's referring to eudaimonia, a concept that he stole from John Locke's Essay Concerning Human Understanding (1690). (