Excellent question!
My definition of happiness is pretty simple..
Being happy means eliminating the things that bother you from your life. For me, there are things that I worry about every waking moment. To free me from this worry, I would be truly happy
What's interesting is, I feel there are different levels of happiness. For example, I would be happier if humanity could alleviate poverty than I would be not worrying about bills..
So I think my definition of happiness kind of hinges on this question; Happiness for me, or happiness in general? Am I free to be as selfish as I wish in this theoretical or are we speaking universally, what would make me the absolute happiest I could attain?