The big question question and should be easy to answer, after all, if we are searching for something, don't we know what it is? How would we know if we found it, assuming it was lost?
I would say happiness isn't one thing or idea or even destination and to put a tag on it and identify it by a name, is probably impossible. Happiness is all about where you are at the moment. What makes me happy is all in relationship to where I am at the moment and that changes all the time.
If we look at happiness as a destination to get to or a ultimate reward and we can stop and enjoy our arrival we missed moments of happiness on the way to achieving something we already have.
If you feel unhappy or on that mission to the ultimate happiness or success (which is to be happy), I would suggest to stop what you are doing and find some small thing you enjoy and makes you happy now and do it. It doesn't have to be life changing, monumental or spectacular but instead some little thing that is accessible and easy you enjoy and just do it. The trick is, pay attention, let yourself have fun without trying to make it anything other than a break to do what you like.
Fill your time with moments like this and before you know it, you may be happy by accident.