If you grant that is possible to "feel happy" (for whatever definition of "happy" you choose), then happiness can be defined the state (or the "emotion") of feeling happy. Sure, happiness is not a concrete entity (though it does have concrete/physical underpinnings in people's brains), so in this sense it doesn't "exist literally", but I don't know where you're going with that. You can still measure it and devise strategies to have people experience more of it.