Defining happiness is oddly difficult, but I'll give it a go.
I'd say happiness is being able to live in and enjoy the moment because there's nothing in your past you'd change, and nothing you anticipate in your future that causes you dread.
When you can look back on every experience and moment of your life and realize that each was essential in making you who you are, and when you can accept that because you like who you are, you're on your way to happiness. When that understanding helps you to embrace tomorrow, regardless of what it might bring, you've achieved happiness.
As I look back and read that, I'm not sure I'm satisfied with that definition, but it's the best I can do at the moment. Maybe after another coffee, I can do better