what is happiness?

Is happiness what you feel in a given moment?  How can we make promises for happiness when we do not know how time will change things.  I don't think happiness is a constant.  But than how do you know if you are unhappy temporarily or worse?  Do things have to change to get better or do you have to change or will things work it self out?  What if everything is a process and not a result?  Than can we really define what happiness is?  Maybe a happy moment is a path to unhappiness or vice versa?