happiness

To me "happiness is best conceived as joy. Joy is unambiguous when experienced. Life can be very terrible; I'm realizing that right now because of a spate of illnesses and deaths I'm observing among my friends. But this terrible quality is not a matter of ugliness, like war; it has the beauty of tragedy (the art form). My losses only emphasize the value of friendship, love, and life. That makes for joy.

Happiness, in my opinion, is relative to an individual’s perspective.

“…but I believe that, as a species, human beings define their happiness through misery and suffering.”
~Agent Smith, The Matrix

Good answers so far.

Happiness is an emotion, and I would say that is the definition. A purely positive emotion.
But what causes happiness?
Only one thing I know of, and that is an individual's ability to experience happiness.
I know some people who find that very hard. If there is nothing to make them miserable, they find something. It seems to me that some people, perhaps without knowing it, seek misery and cannot stand real happiness. I meet them every day, and a paradox is perhaps that those people tend to be more profound and interesting than those who walk around with a great big sun on their forecast for eternity.

according to charles schulz, happiness is a warm puppy

according to john lennon, happiness is a warm gun

Yes, a puppy that doesn't pee on your pillow...