Do you mean definition of happiness, or what happiness means to us (examples of what makes us happy personally)? In terms of definition, happiness can really only be defined ostensively. We cannot define it in terms of other primitives. So when you look up a dictionary, you just find synonyms, or that it's the result of "pleasurable" actions that by definition lead to happiness. This forms a circular definition. Personally, I would say happiness is the feeling which I ultimately always seek. The means through which have varied throughout life, but this common ultimate goal is what unifies my notion of happiness.