What it means to be truly happy has been discussed time and again but, to me, happiness means peace.
And peace to me means lack of stress. It means being able to learn through fulfilling work knowing the deadline (if any) is far in advance, or being able to take a walk knowing that the most hassle you're dealing with that day is making a cup of coffee. It means being able to seamlessly focus on the present without worrying about the past or the irrational uncertainties of the future. It means being able to workout knowing your body is healthy enough to workout again the next day. It means being able to grab lunch with a best friend.
It means waking up in the morning knowing you'll be going to bed the following night.