Knowing what it is and having a concise definition for it does not have to be the same thing, I think. I just finished reading Mindfullness in Plain English, in which Bhante Henepola Gunaratana wrote the following passage:

MINDFULNESS is the English translation of the Pali word sati. Sati is an activity. What exactly is that? There can be no precise answer, at least not in words. Words are devised by the symbolic levels of the mind, and they describe those realities with which symbolic thinking deals. Mindfulness is presymbolic. It is not shackled to logic. Nevertheless, mindfulness can be experienced—rather easily—and it can be described, as long as you keep in mind that the words are only fingers pointing at the moon. They are not the moon itself. The actual experience lies beyond the words and above the symbols. Mindfulness could be described in completely different terms than will be used here, and each description could still be correct.

Remove the bit about Pali, and swap out mindfulness for $MENTAL_STATE. Language is great, but it isn't infallible.

When I would say I am happy: When I'm not feeling lots of pain, apathy, fear, sadness, or anger. When I'm aware that I'm not feeling losts of pain, apathy, fear, sadness, or anger. I'm more likely to be happy when I'm doing something I enjoy, but not always. And doing something I dislike doesn't necessarily preclude happiness. Probably a shitty response, but the more I try to drill down and get specific, the more disagree with my answer.

Having healthy relationships definitly fosters my own happiness. So does acting to address problems that I perceive and am able to influcence.

I find my own happiness falls closer to contentment than excitement.