I gave that some though too. Short and simple definitions are best, so how about the power of choice? The ability to choose is broad enough to cover all kinds of things and fundamental enough to matter.
Do we really have choice? http://exploringthemind.com/the-mind/brain-scans-can-reveal-... I would argue that every choice you make has already be taken before you think it by your subconsciousness which has been programmed to respond to the environment. Just like you react to heat and cold pain and hunger all choices you make are a response to the environment.
I have a lot of issues with that study and that article, but I can put them aside. I did not argue that we are happy, or that we are free -- that's a separate debate. I do think, however, that freedom (in a grand sense) is the path to happiness (in the abstract).
Happiness is probably a chemical state a balance triggered by things like the perception of freedom and choice. IE having food shelter positive social experiences and psychological well being. I think happiness isn't so much a goal as a side effect or something like the opposite of a stressed state.