Happiness and unhappiness are opposite sides of a judgement about your situation. If you judge your situation as bad for you, that's unhappiness. If you judge a situation as good for you, that's happiness.
The experience of happiness is one of those general terms we use to say, "I feel good emotionally." People use different terms to describe what feels good for them. For someone it might be excitement, passion, exhilaration, fulfillment, freedom, feeling fully alive with inspiration and joy. For another it might be more peaceful, content, capable, hopeful, satisfied, and comfortable feeling. Whatever you call it, it just feels darn good.
Our natural state of being is to be happy. When you remove all the uncomfortable emotions we humans can experience (and they are numerous), you're left with happiness. So it's easiest to define happiness by what it is not.
Happiness is what you feel when you're NOT feeling....