Personally, I define happiness as being content with where you are right now - you wouldn't want to be in any other place, in any other time, in any other state of mind. You're happy here, right now. You have your inner peace.
I was listening to a podcast some time ago, and one quote struck a chord with me: “Desire is a contract you make with yourself to be unhappy until you get what you want”.
Perhaps happiness is a choice after all.